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<div class="titlepage"><div><div><h3 class="title">
<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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<p>
        In case you observe a failure in unit tests and you are using a debugger
        to determine the cause, it may get really difficult to step into the expression
        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
        tree before evaluating it, the "Step Into" function of the debugger
        will have to step into every step of building the expression tree before,
        you can go into the evaluation of the expression.
      </p>
<p>
        In order to mitigate the problem, the test module can be build in the mode
        which disables the building of expression trees inside assertions. In this
        mode, the first thing the assertion does is to eagerly evaluate the tested
        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>
        (either with <code class="computeroutput"><span class="preprocessor">#define</span></code> or
        with compiler option <code class="computeroutput"><span class="special">-</span><span class="identifier">D</span></code>)
        prior to including any of the <span class="emphasis"><em>Unit Test Framework</em></span> headers.
      </p>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
          When the eager evaluation of expressions is turned on, the expressions
          are evaluated <span class="emphasis"><em>literally</em></span>: this automatically disables
          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>
          versions of sequence comparisons. This may turn passing assertions into
          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>
          comparisons of sequences, it may render an ill-formed program, if the sequences
          of different types are being compared.
        </p></td></tr>
</table></div>
<p>
        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>
        is that you have to recompile the test module. The <span class="emphasis"><em>Unit Test Framework</em></span>
        gives you another option to compile two versions of the assertions and select
        the one to be used dynamically depending on whether the test module is run
        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
        with <code class="computeroutput"><span class="preprocessor">#define</span></code> or with compiler
        option <code class="computeroutput"><span class="special">-</span><span class="identifier">D</span></code>)
        prior to the inclusion of any of the <span class="emphasis"><em>Unit Test Framework</em></span>
        headers.
      </p>
<p>
        In order to determine if the test module is run under debugger or not, 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>
        is used.
      </p>
<div class="caution"><table border="0" summary="Caution">
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
          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
          the debugger only on MSVC and a few Linux variants.
        </p></td></tr>
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