107 lines
7.2 KiB
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107 lines
7.2 KiB
HTML
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<title>THE BOOST MPL LIBRARY: Exercises</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./details.html" class="navigation-link">Prev</a> <a href="./reference-manual.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./details.html" class="navigation-link">Back</a> Along</span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial-metafunctions.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./exercises.html" class="navigation-link">Exercises</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="exercises">
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<h1><a class="toc-backref" href="./tutorial-metafunctions.html#id60" name="exercises">Exercises</a></h1>
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<dl>
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<dt>1-0.</dt>
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<dd>Use <tt class="literal"><span class="pre">BOOST_STATIC_ASSERT</span></tt> to add error checking to the <tt class="literal"><span class="pre">binary</span></tt>
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template presented in section <a class="reference" href="./resources.html">1</a>.4.1 so
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that <tt class="literal"><span class="pre">binary<N>::value</span></tt> causes a compilation error if <tt class="literal"><span class="pre">N</span></tt>
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contains digits other than <tt class="literal"><span class="pre">0</span></tt> or <tt class="literal"><span class="pre">1</span></tt>.</dd>
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<dt>1-1.</dt>
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<dd>Turn <tt class="literal"><span class="pre">vector_c<int,1,2,3></span></tt> into a type sequence with elements
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(2,3,4) using <tt class="literal"><span class="pre">transform</span></tt>.</dd>
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<dt>1-2.</dt>
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<dd>Turn <tt class="literal"><span class="pre">vector_c<int,1,2,3></span></tt> into a type sequence with elements
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(1,4,9) using <tt class="literal"><span class="pre">transform</span></tt>.</dd>
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<dt>1-3.</dt>
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<dd>Turn <tt class="literal"><span class="pre">T</span></tt> into <tt class="literal"><span class="pre">T****</span></tt> by using <tt class="literal"><span class="pre">twice</span></tt> twice.</dd>
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<dt>1-4.</dt>
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<dd>Turn <tt class="literal"><span class="pre">T</span></tt> into <tt class="literal"><span class="pre">T****</span></tt> using <tt class="literal"><span class="pre">twice</span></tt> on itself.</dd>
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<dt>1-5.</dt>
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<dd><p class="first">There's still a problem with the dimensional analysis code in
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section 1.1.
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Hint: What happens when you do:</p>
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<pre class="literal-block">
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f = f + m * a;
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</pre>
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<p>Repair this example using techniques shown in this
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chapter.</p>
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<!-- @ example.wrap('''void will_fail%s
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{ quantity<float,force> f(m*a);
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''' % ma_function_args, '}')
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stack[:0] = dimensional_analysis # stick support code in
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compile('all', expect_error = True) -->
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</dd>
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<dt>1-6.</dt>
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<dd>Build a lambda expression that has functionality equivalent to
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<tt class="literal"><span class="pre">twice</span></tt>. Hint: <tt class="literal"><span class="pre">mpl::apply</span></tt> is a metafunction!</dd>
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<dt>1-7*.</dt>
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<dd><p class="first">What do you think would be the semantics of the following
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constructs:</p>
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<pre class="literal-block">
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typedef mpl::lambda<mpl::lambda<_1> >::type t1;
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typedef mpl::apply<_1,mpl::plus<_1,_2> >::type t2;
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typedef mpl::apply<_1,std::vector<int> >::type t3;
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typedef mpl::apply<_1,std::vector<_1> >::type t4;
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typedef mpl::apply<mpl::lambda<_1>,std::vector<int> >::type t5;
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typedef mpl::apply<mpl::lambda<_1>,std::vector<_1> >::type t6;
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typedef mpl::apply<mpl::lambda<_1>,mpl::plus<_1,_2> >::type t7;
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typedef mpl::apply<_1,mpl::lambda< mpl::plus<_1,_2> > >::type t8;
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</pre>
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<!-- @example.prepend('#include <vector>')
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compile() -->
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<p class="last">Show the steps used to
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arrive at your answers and write tests verifying your assumptions.
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Did the library behavior match your reasoning? If not, analyze the
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failed tests to discover the actual expression semantics.
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Explain why your assumptions were different, what
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behavior you find more coherent, and why.</p>
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</dd>
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<dt>1-8*.</dt>
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<dd>Our dimensional analysis framework dealt with dimensions, but it
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entirely ignored the issue of <em>units</em>. A length can be
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represented in inches, feet, or meters. A force can be
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represented in newtons or in kg m/sec<sup>2</sup>. Add the
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ability to specify units and test your code. Try to make your
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interface as syntactically friendly as possible for the user.</dd>
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</dl>
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<!-- Along with Alan Turing, Church was one of the founders of computer
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science. -->
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<!-- yes, this is easier to read, but the real beauty of ``apply`` is that
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it not only works on metafunction classes, but on metafunctions passed
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with placeholder arguments. -->
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<!-- Naturally, MPL defines a series of these numbered placeholder
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types, so that we can handle metafunctions with more arguments. -->
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<!-- : in order to pass a template in the ``BinaryOperation`` position,
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``transform`` would have to be declared::
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template <
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class Sequence1, class Sequence2
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, template <class,class> class BinaryOperation
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>
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struct transform; // returning a sequence -->
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<!-- "Programming with types" -->
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<!-- Once again, a nested ``value`` member is optional for
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metafunctions, and required for integral type wrappers. -->
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</div>
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