ASSERTION
An assertion statement specifies a condition that you expect to hold true at some particular point in your program. If that condition does not hold true, the assertion fails, execution of your program is interrupted, and the Assertion Failed dialog box appears.
MFC and C Run-Time Library Assertions
When the debugger halts because of an MFC or C run-time library assertion, it navigates to the point in the source file where the assertion occurred (if the source is available). The assertion message appears in the Output window as well as the Assertion Failed dialog box. You can copy the assertion message from the Output window to a text window if you want to save it for future reference. The Output window may contain other error messages as well. Examine these messages carefully, because they provide clues to the cause of the assertion failure.
_DEBUG
Assertion statements compile only when _DEBUG is defined. When _DEBUG is not defined, the compiler treats assertions as null statements. Therefore, assertion statements have zero overhead in your final release program; you can use them liberally in your code without affecting the performance of your Release version and without having to use #ifdefs.
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