ae01c8387c
- section about command line argument filtering for template test cases - many typos fixes - remove reference to bjam
79 lines
3.4 KiB
Plaintext
79 lines
3.4 KiB
Plaintext
[/
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/ Copyright (c) 2015 Boost.Test contributors
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/
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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/]
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[section:internal_details `BOOST_TEST`: details on expressions]
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Let's consider the following example:
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[bt_example boost_test_macro3..BOOST_TEST reporting..run-fail]
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It was already mentioned that the reporting is not symmetrical (see [link boost_test.testing_tools.reports here]).
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An expression is constructed from the `statement` appearing in the `BOOST_TEST` macro. This expression allows evaluation and reporting such
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as `"13 - 1 >= 12" failed` along with a copy of the `statement`, which contains more details than `"a - 1 < b" failed`.
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In details, what happens is the following:
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# a special object, the `seed` of the expression, is composed from the left side of `statement`.
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This initial composition has highest precedence over the supported operations. The expression below:
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a op1 b op2 c op3 d
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is actually seen as
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( seed a ) op1 b op2 c op3 d
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# The "`seed a`" returns an `expression` object that keep tracks of the type of `a`. This expression
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has overloads for left-to-right associativity, and the
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operations `op1`, `op2` ... are /chained/ to the right of this expression object:
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a op1 b
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yields to the pseudo-code
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expression1 = create-expression(a)
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expression2 = create-expression(expression1, op1, b)
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`expression1` and `expression2` keep track of their left and right operands, and the operation on those operands. The
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expressions keep also track of the result type of the associated sub-expression. In the above example, `expression1` and `expression2`
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have result type `decltype(a)` and `decltype(a op1 b)` respectively. The result type allows for chaining
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sub-expressions.
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# The C++ operators precedence rules apply in any case. What is seen by the expression is what is reachable with left-to-right
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composition. Any other operation that happens before it reaches the expression's right operand is not parsed as a sub-expression
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and is seen as a single operand: the right operand is not developed further by the framework.
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Let's suppose `op2` below has higher precedence than `op1`, then
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a op1 b op2 c
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is equivalent to:
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create-expression(create-expression(a), op1, (b op2 c))
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In the above statement, the final expression can only see the result of `(b op2 c)` to its right, for which no further detail
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can be provided in the logs. This is also the case for /right-to-left/ associative operators, such as `!`, `~`, `-` (unary negation)
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etc.
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[caution Since the `expression` object is composed from left-to-right, it actually observes a chain of operations and
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not the full expression tree.]
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# Once the full expression chain is built, it is evaluated as a chain of sub-expressions from left-to-right, exactly as the
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composition rule above. The evaluated elements are the ones of the expression itself. The expression
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a op1 b
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yields to the following evaluation chain:
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expression2.result = expression1.result op1 b
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expression1.result = a
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The final expression of the statement is cast to a boolean, which is in turn evaluated by the __UTF__.
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The example below illustrates the construction of the left-to-right /chained/ expression.
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[bt_example boost_test_macro2..BOOST_TEST compound statements..run-fail]
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[endsect]
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