102 lines
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102 lines
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<title>Debugging the assertions</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_test.testing_tools.debugging"></a><a class="link" href="debugging.html" title="Debugging the assertions">Debugging the assertions</a>
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</h3></div></div></div>
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<p>
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In case you observe a failure in unit tests and you are using a debugger
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to determine the cause, it may get really difficult to step into the expression
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inside an assertion. Because <a class="link" href="../utf_reference/testing_tool_ref/assertion_boost_test_universal_macro.html" title="BOOST_TEST"><code class="computeroutput"><span class="identifier">BOOST_TEST</span></code></a> builds an expression
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tree before evaluating it, the "Step Into" function of the debugger
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will have to step into every step of building the expression tree before,
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you can go into the evaluation of the expression.
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</p>
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<p>
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In order to mitigate the problem, the test module can be build in the mode
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which disables the building of expression trees inside assertions. In this
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mode, the first thing the assertion does is to eagerly evaluate the tested
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expression. You enable this mode by defining symbol <a class="link" href="../utf_reference/testing_tool_ref/assertion_control_under_debugger.html" title="BOOST_TEST_TOOLS_UNDER_DEBUGGER"><code class="computeroutput"><span class="identifier">BOOST_TEST_TOOLS_UNDER_DEBUGGER</span></code></a>
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(either with <code class="computeroutput"><span class="preprocessor">#define</span></code> or
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with compiler option <code class="computeroutput"><span class="special">-</span><span class="identifier">D</span></code>)
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prior to including any of the <span class="emphasis"><em>Unit Test Framework</em></span> headers.
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</p>
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<div class="caution"><table border="0" summary="Caution">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
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<th align="left">Caution</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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When the eager evaluation of expressions is turned on, the expressions
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are evaluated <span class="emphasis"><em>literally</em></span>: this automatically disables
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any special semantics, like tolerance for floating-point types or <code class="computeroutput"><a class="link" href="../../boost/test_tools/per_element.html" title="Struct per_element">boost::test_tools::per_element</a></code>
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versions of sequence comparisons. This may turn passing assertions into
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failing assertions and vice-versa. In the case of <code class="computeroutput"><a class="link" href="../../boost/test_tools/per_element.html" title="Struct per_element">boost::test_tools::per_element</a></code>
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comparisons of sequences, it may render an ill-formed program, if the sequences
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of different types are being compared.
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</p></td></tr>
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</table></div>
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<p>
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The inconvenience with <a class="link" href="../utf_reference/testing_tool_ref/assertion_control_under_debugger.html" title="BOOST_TEST_TOOLS_UNDER_DEBUGGER"><code class="computeroutput"><span class="identifier">BOOST_TEST_TOOLS_UNDER_DEBUGGER</span></code></a>
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is that you have to recompile the test module. The <span class="emphasis"><em>Unit Test Framework</em></span>
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gives you another option to compile two versions of the assertions and select
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the one to be used dynamically depending on whether the test module is run
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under debugger or not. This mode is enabled by defining symbol <a class="link" href="../utf_reference/testing_tool_ref/assertion_control_under_debuggable.html" title="BOOST_TEST_TOOLS_DEBUGGABLE"><code class="computeroutput"><span class="identifier">BOOST_TEST_TOOLS_DEBUGGABLE</span></code></a> (either
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with <code class="computeroutput"><span class="preprocessor">#define</span></code> or with compiler
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option <code class="computeroutput"><span class="special">-</span><span class="identifier">D</span></code>)
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prior to the inclusion of any of the <span class="emphasis"><em>Unit Test Framework</em></span>
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headers.
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</p>
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<p>
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In order to determine if the test module is run under debugger or not, function
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<a class="link" href="../../boost/debug/under_debugger.html" title="Function under_debugger"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">debug</span><span class="special">::</span><span class="identifier">under_debugger</span></code></a>
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is used.
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</p>
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<div class="caution"><table border="0" summary="Caution">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
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<th align="left">Caution</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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At present, function <a class="link" href="../../boost/debug/under_debugger.html" title="Function under_debugger"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">debug</span><span class="special">::</span><span class="identifier">under_debugger</span></code></a> can correctly detect
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the debugger only on MSVC and a few Linux variants.
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</p></td></tr>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2017 Boost.Test contributors<p>
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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 <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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