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 BOOST_TEST 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.

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 BOOST_TEST_TOOLS_UNDER_DEBUGGER (either with #define or with compiler option -D) prior to including any of the Unit Test Framework headers.

Caution

When the eager evaluation of expressions is turned on, the expressions are evaluated literally: this automatically disables any special semantics, like tolerance for floating-point types or boost::test_tools::per_element versions of sequence comparisons. This may turn passing assertions into failing assertions and vice-versa. In the case of boost::test_tools::per_element comparisons of sequences, it may render an ill-formed program, if the sequences of different types are being compared.

The inconvenience with BOOST_TEST_TOOLS_UNDER_DEBUGGER is that you have to recompile the test module. The Unit Test Framework 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 BOOST_TEST_TOOLS_DEBUGGABLE (either with #define or with compiler option -D) prior to the inclusion of any of the Unit Test Framework headers.

In order to determine if the test module is run under debugger or not, function boost::debug::under_debugger is used.

	Caution
	At present, function boost::debug::under_debugger can correctly detect the debugger only on MSVC and a few Linux variants.