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