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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_multiprecision.tut.limits.functions"></a><a class="link" href="functions.html" title="std::numeric_limits&lt;&gt; functions"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;</span></code> functions</a>
</h4></div></div></div>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h0"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.max_function"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.max_function"><code class="computeroutput"><span class="identifier">max</span></code> function</a>
</h5>
<p>
Function <code class="computeroutput"><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">)()</span></code> returns the largest finite value that
can be represented by the type T. If there is no such value (and <code class="computeroutput"><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">bounded</span></code> is <code class="computeroutput"><span class="keyword">false</span></code>)
then returns <code class="computeroutput"><span class="identifier">T</span><span class="special">()</span></code>.
</p>
<p>
For built-in types there is usually a corresponding MACRO value TYPE_MAX,
where TYPE is CHAR, INT, FLOAT etc.
</p>
<p>
Other types, including those provided by a typedef, for example <code class="computeroutput"><span class="identifier">INT64_T_MAX</span></code> for <code class="computeroutput"><span class="identifier">int64_t</span></code>,
may provide a macro definition.
</p>
<p>
To cater for situations where no <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
specialization is available (for example because the precision of the type
varies at runtime), packaged versions of this (and other functions) are
provided using
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">tools</span><span class="special">/</span><span class="identifier">precision</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="identifier">T</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">::</span><span class="identifier">max_value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;();</span>
</pre>
<p>
Of course, these simply use <code class="computeroutput"><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">)()</span></code>
if available, but otherwise 'do something sensible'.
</p>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h1"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.lowest_function"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.lowest_function">lowest
function</a>
</h5>
<p>
Since C++11: <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">lowest</span><span class="special">()</span></code>
is
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
For integral types, the same as function <code class="computeroutput"><span class="identifier">min</span><span class="special">()</span></code>.
</li>
<li class="listitem">
For floating-point types, generally the negative of <code class="computeroutput"><span class="identifier">max</span><span class="special">()</span></code> (but implementation-dependent).
</li>
</ul></div>
<pre class="programlisting"><span class="special">-(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">)()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">lowest</span><span class="special">();</span>
</pre>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h2"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.min_function"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.min_function"><code class="computeroutput"><span class="identifier">min</span></code> function</a>
</h5>
<p>
Function <code class="computeroutput"><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">min</span><span class="special">)()</span></code> returns the minimum finite value that
can be represented by the type T.
</p>
<p>
For built-in types, there is usually a corresponding MACRO value TYPE_MIN,
where TYPE is CHAR, INT, FLOAT etc.
</p>
<p>
Other types, including those provided by a <code class="computeroutput"><span class="keyword">typedef</span></code>,
for example, <code class="computeroutput"><span class="identifier">INT64_T_MIN</span></code>
for <code class="computeroutput"><span class="identifier">int64_t</span></code>, may provide
a macro definition.
</p>
<p>
For floating-point types, it is more fully defined as the <span class="emphasis"><em>minimum
positive normalized value</em></span>.
</p>
<p>
See <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">denorm_min</span><span class="special">()</span></code>
for the smallest denormalized value, provided
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">has_denorm</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">denorm_present</span>
</pre>
<p>
To cater for situations where no <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
specialization is available (for example because the precision of the type
varies at runtime), packaged versions of this (and other functions) are
provided using
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">tools</span><span class="special">/</span><span class="identifier">precision</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="identifier">T</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">::</span><span class="identifier">min_value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;();</span>
</pre>
<p>
Of course, these simply use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">min</span><span class="special">()</span></code> if available.
</p>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h3"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.denorm_min_function"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.denorm_min_function">denorm_min
function</a>
</h5>
<p>
Function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">denorm_min</span><span class="special">()</span></code>
returns the smallest <a href="http://en.wikipedia.org/wiki/Denormal_number" target="_top">denormalized
value</a>, provided
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">has_denorm</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">denorm_present</span>
</pre>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">.</span><span class="identifier">precision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">has_denorm</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">denorm_present</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">denorm_min</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// 4.9406564584124654e-324</span>
<span class="keyword">int</span> <span class="identifier">exponent</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">significand</span> <span class="special">=</span> <span class="identifier">frexp</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">exponent</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"exponent = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span> <span class="special">&lt;&lt;</span> <span class="identifier">exponent</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// fffffbcf</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"significand = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span> <span class="special">&lt;&lt;</span> <span class="identifier">significand</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// 0.50000000000000000</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"No denormalization. "</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
The exponent is effectively reduced from -308 to -324 (though it remains
encoded as zero and leading zeros appear in the significand, thereby losing
precision until the significand reaches zero).
</p>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h4"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.round_error"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.round_error">round_error</a>
</h5>
<p>
Function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">round_error</span><span class="special">()</span></code>
returns the maximum error (in units of <a href="http://en.wikipedia.org/wiki/Unit_in_the_last_place" target="_top">ULP</a>)
that can be caused by any basic arithmetic operation.
</p>
<pre class="programlisting"><span class="identifier">round_style</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">round_indeterminate</span><span class="special">;</span>
</pre>
<p>
The rounding style is indeterminable at compile time.
</p>
<p>
For floating-point types, when rounding is to nearest, only half a bit
is lost by rounding, and <code class="computeroutput"><span class="identifier">round_error</span>
<span class="special">==</span> <span class="number">0.5</span></code>.
In contrast when rounding is towards zero, or plus/minus infinity, we can
loose up to one bit from rounding, and <code class="computeroutput"><span class="identifier">round_error</span>
<span class="special">==</span> <span class="number">1</span></code>.
</p>
<p>
For integer types, rounding always to zero, so at worst almost one bit
can be rounded, so <code class="computeroutput"><span class="identifier">round_error</span>
<span class="special">==</span> <span class="number">1</span></code>.
</p>
<p>
<code class="computeroutput"><span class="identifier">round_error</span><span class="special">()</span></code>
can be used with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">()</span></code>
to estimate the maximum potential error caused by rounding. For typical
floating-point types, <code class="computeroutput"><span class="identifier">round_error</span><span class="special">()</span> <span class="special">=</span> <span class="number">1</span><span class="special">/</span><span class="number">2</span></code>, so half
epsilon is the maximum potential error.
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">round_err</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">()</span> <span class="comment">// 2.2204460492503131e-016</span>
<span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">round_error</span><span class="special">();</span> <span class="comment">// 1/2</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">round_err</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// 1.1102230246251565e-016</span>
</pre>
<p>
There are, of course, many occasions when much bigger loss of precision
occurs, for example, caused by <a href="http://en.wikipedia.org/wiki/Loss_of_significance" target="_top">Loss
of significance or cancellation error</a> or very many iterations.
</p>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h5"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.epsilon"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.epsilon">epsilon</a>
</h5>
<p>
Function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">()</span></code>
is meaningful only for non-integral types.
</p>
<p>
It returns the difference between <code class="computeroutput"><span class="number">1.0</span></code>
and the next value representable by the floating-point type T. So it is
a one least-significant-bit change in this floating-point value.
</p>
<p>
For <code class="computeroutput"><span class="keyword">double</span></code> (<code class="computeroutput"><span class="identifier">float_64t</span></code>) it is <code class="computeroutput"><span class="number">2.2204460492503131e-016</span></code>
showing all possibly significant 17 decimal digits.
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">.</span><span class="identifier">precision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">);</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">eps</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
<span class="keyword">double</span> <span class="identifier">dpeps</span> <span class="special">=</span> <span class="identifier">d</span><span class="special">+</span><span class="identifier">eps</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">showpoint</span> <span class="comment">// Ensure all trailing zeros are shown.</span>
<span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span> <span class="comment">// 1.0000000000000000</span>
<span class="special">&lt;&lt;</span> <span class="identifier">dpeps</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// 2.2204460492503131e-016</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">dpeps</span> <span class="special">-</span> <span class="identifier">d</span> <span class="comment">// 1.0000000000000002</span>
<span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
We can explicitly increment by one bit using the function <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">float_next</span><span class="special">()</span></code>
and the result is the same as adding <code class="computeroutput"><span class="identifier">epsilon</span></code>.
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">one</span> <span class="special">=</span> <span class="number">1.</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">nad</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">float_next</span><span class="special">(</span><span class="identifier">one</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">nad</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span> <span class="comment">// 1.0000000000000002</span>
<span class="special">&lt;&lt;</span> <span class="identifier">nad</span> <span class="special">-</span> <span class="identifier">one</span> <span class="comment">// 2.2204460492503131e-016</span>
<span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
Adding any smaller value, like half <code class="computeroutput"><span class="identifier">epsilon</span></code>,
will have no effect on this value.
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">.</span><span class="identifier">precision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">);</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">eps</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
<span class="keyword">double</span> <span class="identifier">dpeps</span> <span class="special">=</span> <span class="identifier">d</span> <span class="special">+</span> <span class="identifier">eps</span><span class="special">/</span><span class="number">2</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">showpoint</span> <span class="comment">// Ensure all trailing zeros are shown.</span>
<span class="special">&lt;&lt;</span> <span class="identifier">dpeps</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span> <span class="comment">// 1.0000000000000000</span>
<span class="special">&lt;&lt;</span> <span class="identifier">eps</span><span class="special">/</span><span class="number">2</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// 1.1102230246251565e-016</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">dpeps</span> <span class="special">-</span> <span class="identifier">d</span> <span class="comment">// 0.00000000000000000</span>
<span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
So this cancellation error leaves the values equal, despite adding half
<code class="computeroutput"><span class="identifier">epsilon</span></code>.
</p>
<p>
To achieve greater portability over platform and floating-point type, Boost.Math
and Boost.Multiprecision provide a package of functions that 'do something
sensible' if the standard <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
is not available. To use these <code class="computeroutput"><span class="preprocessor">#include</span>
<span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">tools</span><span class="special">/</span><span class="identifier">precision</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
</p>
<p>
A tolerance might be defined using this version of epsilon thus:
</p>
<pre class="programlisting"><span class="identifier">RealType</span> <span class="identifier">tolerance</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">::</span><span class="identifier">epsilon</span><span class="special">&lt;</span><span class="identifier">RealType</span><span class="special">&gt;()</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span>
</pre>
<h6>
<a name="boost_multiprecision.tut.limits.functions.h6"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.FP_tolerance"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.FP_tolerance">Tolerance
for Floating-point Comparisons</a>
</h6>
<p>
<a href="https://en.wikipedia.org/wiki/Machine_epsilon" target="_top">Machine epsilon
&#949;</a> is very useful to compute a tolerance when comparing floating-point
values, a much more difficult task than is commonly imagined.
</p>
<p>
The C++ standard specifies <a href="https://en.cppreference.com/w/cpp/types/numeric_limits/epsilon" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;::</span><span class="identifier">epsilon</span><span class="special">()</span></code></a>
and Boost.Multiprecision implements this (where possible) for its program-defined
types analogous to the __fundamental floating-point types like <code class="computeroutput"><span class="keyword">double</span></code> <code class="computeroutput"><span class="keyword">float</span></code>.
</p>
<p>
For more information than you probably want (but still need) see <a href="http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html" target="_top">What
Every Computer Scientist Should Know About Floating-Point Arithmetic</a>
</p>
<p>
The naive test comparing the absolute difference between two values and
a tolerance does not give useful results if the values are too large or
too small.
</p>
<p>
So Boost.Test uses an algorithm first devised by Knuth for reliably checking
if floating-point values are close enough.
</p>
<p>
See Donald. E. Knuth. The art of computer programming (vol II). Copyright
1998 Addison-Wesley Longman, Inc., 0-201-89684-2. Addison-Wesley Professional;
3rd edition. (The relevant equations are in paragraph 4.2.2, Eq. 36 and
37.)
</p>
<p>
See <a href="https://www.boost.org/doc/libs/release/libs/test/doc/html/boost_test/testing_tools/extended_comparison/floating_point/floating_points_comparison_theory.html" target="_top">Boost.Math
floating_point comparison</a> for more details.
</p>
<p>
See also:
</p>
<p>
<a href="http://adtmag.com/articles/2000/03/15/comparing-floats-how-to-determine-if-floating-quantities-are-close-enough-once-a-tolerance-has-been.aspx" target="_top">Alberto
Squassia, Comparing floats</a>
</p>
<p>
<a href="http://adtmag.com/articles/2000/03/16/comparing-floats-how-to-determine-if-floating-quantities-are-close-enough-once-a-tolerance-has-been.aspx" target="_top">Alberto
Squassia, Comparing floats code</a>
</p>
<p>
<a href="https://www.boost.org/doc/libs/release/libs/test/doc/html/boost_test/testing_tools/extended_comparison/floating_point.html" target="_top">Boost.Test
Floating-Point_Comparison</a>
</p>
<p>
For example, if we want a tolerance that might suit about 9 arithmetical
operations, say sqrt(9) = 3, we could define:
</p>
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">tolerance</span> <span class="special">=</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
</pre>
<p>
This is very widely used in Boost.Math testing with Boost.Test's macro
<code class="computeroutput"><span class="identifier">BOOST_CHECK_CLOSE_FRACTION</span></code>
</p>
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">expected</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
<span class="identifier">T</span> <span class="identifier">calculated</span> <span class="special">=</span> <span class="number">1.0</span> <span class="special">+</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
<span class="identifier">BOOST_CHECK_CLOSE_FRACTION</span><span class="special">(</span><span class="identifier">expected</span><span class="special">,</span> <span class="identifier">calculated</span><span class="special">,</span> <span class="identifier">tolerance</span><span class="special">);</span>
</pre>
<p>
used thus:
</p>
<pre class="programlisting"><span class="identifier">cd</span> <span class="special">./</span><span class="identifier">test</span>
<span class="identifier">BOOST_CHECK_CLOSE_FRACTION</span><span class="special">(</span><span class="identifier">expected</span><span class="special">,</span> <span class="identifier">calculated</span><span class="special">,</span> <span class="identifier">tolerance</span><span class="special">);</span>
</pre>
<p>
(There is also a version BOOST_CHECK_CLOSE using tolerance as a <span class="bold"><strong>percentage</strong></span> rather than a fraction; usually the fraction
version is simpler to use).
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">number</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">cpp_dec_float</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">et_off</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_dec_float</span><span class="special">&lt;</span><span class="number">50</span><span class="special">&gt;,</span> <span class="identifier">et_off</span> <span class="special">&gt;</span> <span class="identifier">cpp_dec_float_50</span><span class="special">;</span> <span class="comment">// 50 decimal digits.</span>
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
that Boost.Test does not yet allow floating-point comparisons with expression
templates on, so the default expression template parameter has been replaced
by <code class="computeroutput"><span class="identifier">et_off</span></code>.
</p></td></tr>
</table></div>
<pre class="programlisting"><span class="identifier">cpp_dec_float_50</span> <span class="identifier">tolerance</span> <span class="special">=</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">cpp_dec_float_50</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
<span class="identifier">cpp_dec_float_50</span> <span class="identifier">expected</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">constants</span><span class="special">::</span><span class="identifier">two_pi</span><span class="special">&lt;</span><span class="identifier">cpp_dec_float_50</span><span class="special">&gt;();</span>
<span class="identifier">cpp_dec_float_50</span> <span class="identifier">calculated</span> <span class="special">=</span> <span class="number">2</span> <span class="special">*</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">constants</span><span class="special">::</span><span class="identifier">pi</span><span class="special">&lt;</span><span class="identifier">cpp_dec_float_50</span><span class="special">&gt;();</span>
<span class="identifier">BOOST_CHECK_CLOSE_FRACTION</span><span class="special">(</span><span class="identifier">expected</span><span class="special">,</span> <span class="identifier">calculated</span><span class="special">,</span> <span class="identifier">tolerance</span><span class="special">);</span>
</pre>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h7"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.infinity"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.infinity">Infinity -
positive and negative</a>
</h5>
<p>
For floating-point types only, for which <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">has_infinity</span>
<span class="special">==</span> <span class="keyword">true</span></code>,
function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">infinity</span><span class="special">()</span></code>
provides an implementation-defined representation for &#8734;.
</p>
<p>
The 'representation' is a particular bit pattern reserved for infinity.
For IEEE754 system (for which <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">is_iec559</span>
<span class="special">==</span> <span class="keyword">true</span></code>)
<a href="http://en.wikipedia.org/wiki/IEEE_754-1985#Positive_and_negative_infinity" target="_top">positive
and negative infinity</a> are assigned bit patterns for all defined
floating-point types.
</p>
<p>
Confusingly, the string resulting from outputting this representation,
is also implementation-defined. And the string that can be input to generate
the representation is also implementation-defined.
</p>
<p>
For example, the output is <code class="computeroutput"><span class="number">1.</span><span class="special">#</span><span class="identifier">INF</span></code>
on Microsoft systems, but <code class="computeroutput"><span class="identifier">inf</span></code>
on most *nix platforms.
</p>
<p>
This implementation-defined-ness has hampered use of infinity (and NaNs)
but Boost.Math and Boost.Multiprecision work hard to provide a sensible
representation for <span class="bold"><strong>all</strong></span> floating-point
types, not just the built-in types, which with the use of suitable facets
to define the input and output strings, makes it possible to use these
useful features portably and including Boost.Serialization.
</p>
<h5>
<a name="boost_multiprecision.tut.limits.functions.h8"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.not_a_number_nan"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.not_a_number_nan">Not-A-Number
NaN</a>
</h5>
<h6>
<a name="boost_multiprecision.tut.limits.functions.h9"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.quiet_nan"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.quiet_nan">Quiet_NaN</a>
</h6>
<p>
For floating-point types only, for which <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">has_quiet_NaN</span>
<span class="special">==</span> <span class="keyword">true</span></code>,
function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">quiet_NaN</span><span class="special">()</span></code>
provides an implementation-defined representation for NaN.
</p>
<p>
<a href="http://en.wikipedia.org/wiki/NaN" target="_top">NaNs</a> are values to
indicate that the result of an assignment or computation is meaningless.
A typical example is <code class="computeroutput"><span class="number">0</span><span class="special">/</span><span class="number">0</span></code> but there are many others.
</p>
<p>
NaNs may also be used, to represent missing values: for example, these
could, by convention, be ignored in calculations of statistics like means.
</p>
<p>
Many of the problems with a representation for <a href="http://en.wikipedia.org/wiki/NaN" target="_top">Not-A-Number</a>
has hampered portable use, similar to those with infinity.
</p>
<p>
NaN can be used with binary multiprecision types like <code class="computeroutput"><span class="identifier">cpp_bin_float_quad</span></code>:
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">cpp_bin_float_quad</span><span class="special">;</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">cpp_bin_float_quad</span><span class="special">&gt;::</span><span class="identifier">has_quiet_NaN</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">cpp_bin_float_quad</span> <span class="identifier">tolerance</span> <span class="special">=</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">cpp_bin_float_quad</span><span class="special">&gt;::</span><span class="identifier">epsilon</span><span class="special">();</span>
<span class="identifier">cpp_bin_float_quad</span> <span class="identifier">NaN</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">cpp_bin_float_quad</span><span class="special">&gt;::</span><span class="identifier">quiet_NaN</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"cpp_bin_float_quad NaN is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">NaN</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// cpp_bin_float_quad NaN is nan</span>
<span class="identifier">cpp_bin_float_quad</span> <span class="identifier">expected</span> <span class="special">=</span> <span class="identifier">NaN</span><span class="special">;</span>
<span class="identifier">cpp_bin_float_quad</span> <span class="identifier">calculated</span> <span class="special">=</span> <span class="number">2</span> <span class="special">*</span> <span class="identifier">NaN</span><span class="special">;</span>
<span class="comment">// Comparisons of NaN's always fail:</span>
<span class="keyword">bool</span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">expected</span> <span class="special">==</span> <span class="identifier">calculated</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">BOOST_CHECK_NE</span><span class="special">(</span><span class="identifier">expected</span><span class="special">,</span> <span class="identifier">expected</span><span class="special">);</span>
<span class="identifier">BOOST_CHECK_NE</span><span class="special">(</span><span class="identifier">expected</span><span class="special">,</span> <span class="identifier">calculated</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Type "</span> <span class="special">&lt;&lt;</span> <span class="keyword">typeid</span><span class="special">(</span><span class="identifier">cpp_bin_float_quad</span><span class="special">).</span><span class="identifier">name</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="string">" does not have NaNs!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
But using Boost.Math and suitable facets can permit portable use of both
NaNs and positive and negative infinity.
</p>
<p>
See <a href="../../../../../../../libs/math/example/nonfinite_facet_sstream.cpp" target="_top">boost:/libs/math/example/nonfinite_facet_sstream.cpp</a>
and we also need
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">special_functions</span><span class="special">/</span><span class="identifier">nonfinite_num_facets</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
Then we can equally well use a multiprecision type cpp_bin_float_quad:
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">cpp_bin_float_quad</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">cpp_bin_float_quad</span> <span class="identifier">T</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">nonfinite_num_put</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">nonfinite_num_get</span><span class="special">;</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">old_locale</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">tmp_locale</span><span class="special">(</span><span class="identifier">old_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_put</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">new_locale</span><span class="special">(</span><span class="identifier">tmp_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_get</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span>
<span class="identifier">ss</span><span class="special">.</span><span class="identifier">imbue</span><span class="special">(</span><span class="identifier">new_locale</span><span class="special">);</span>
<span class="identifier">T</span> <span class="identifier">inf</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">infinity</span><span class="special">();</span>
<span class="identifier">ss</span> <span class="special">&lt;&lt;</span> <span class="identifier">inf</span><span class="special">;</span> <span class="comment">// Write out.</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">==</span> <span class="string">"inf"</span><span class="special">);</span>
<span class="identifier">T</span> <span class="identifier">r</span><span class="special">;</span>
<span class="identifier">ss</span> <span class="special">&gt;&gt;</span> <span class="identifier">r</span><span class="special">;</span> <span class="comment">// Read back in.</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">inf</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">);</span> <span class="comment">// Confirms that the floating-point values really are identical.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"infinity output was "</span> <span class="special">&lt;&lt;</span> <span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"infinity input was "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<pre class="programlisting"><span class="identifier">infinity</span> <span class="identifier">output</span> <span class="identifier">was</span> <span class="identifier">inf</span>
<span class="identifier">infinity</span> <span class="identifier">input</span> <span class="identifier">was</span> <span class="identifier">inf</span>
</pre>
<p>
Similarly we can do the same with NaN (except that we cannot use <code class="computeroutput"><span class="identifier">assert</span></code> (because any comparisons with
NaN always return false).
</p>
<pre class="programlisting"><span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">old_locale</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">tmp_locale</span><span class="special">(</span><span class="identifier">old_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_put</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span> <span class="identifier">new_locale</span><span class="special">(</span><span class="identifier">tmp_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_get</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span>
<span class="identifier">ss</span><span class="special">.</span><span class="identifier">imbue</span><span class="special">(</span><span class="identifier">new_locale</span><span class="special">);</span>
<span class="identifier">T</span> <span class="identifier">n</span><span class="special">;</span>
<span class="identifier">T</span> <span class="identifier">NaN</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">quiet_NaN</span><span class="special">();</span>
<span class="identifier">ss</span> <span class="special">&lt;&lt;</span> <span class="identifier">NaN</span><span class="special">;</span> <span class="comment">// Write out.</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">==</span> <span class="string">"nan"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"NaN output was "</span> <span class="special">&lt;&lt;</span> <span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">ss</span> <span class="special">&gt;&gt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="comment">// Read back in.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"NaN input was "</span> <span class="special">&lt;&lt;</span> <span class="identifier">n</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<pre class="programlisting"><span class="identifier">NaN</span> <span class="identifier">output</span> <span class="identifier">was</span> <span class="identifier">nan</span>
<span class="identifier">NaN</span> <span class="identifier">input</span> <span class="identifier">was</span> <span class="identifier">nan</span>
</pre>
<h6>
<a name="boost_multiprecision.tut.limits.functions.h10"></a>
<span class="phrase"><a name="boost_multiprecision.tut.limits.functions.signaling_nan"></a></span><a class="link" href="functions.html#boost_multiprecision.tut.limits.functions.signaling_nan">Signaling
NaN</a>
</h6>
<p>
For floating-point types only, for which <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">has_signaling_NaN</span>
<span class="special">==</span> <span class="keyword">true</span></code>,
function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">signaling_NaN</span><span class="special">()</span></code>
provides an implementation-defined representation for NaN that causes a
hardware trap. It should be noted however, that at least one implementation
of this function causes a hardware trap to be triggered simply by calling
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">signaling_NaN</span><span class="special">()</span></code>,
and not only by using the value returned.
</p>
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