The definition of the macroassert depends on another macro,NDEBUG, which is not defined by the standard library.

IfNDEBUG is defined as a macro name at the point in the source code where<assert.h> is included, thenassert does nothing.

IfNDEBUG is not defined, thenassert checks ifits argument(until C23)the expression synthesized from__VA_ARGS__(since C23) (which must have scalar type, otherwise, the behavior is undefined) compares equal to zero. If it does,assert outputs implementation-specific diagnostic information on the standard error output and callsabort(). The diagnostic information is required to include the text ofexpression, as well as the values of thepredefined variable__func__ and of(since C99) thepredefined macros__FILE__ and__LINE__.

[edit]Parameters

[edit]Return value

(none)

[edit]Notes

There is no standardized interface to add an additional message toassert errors. A portable way to include one is to use acomma operator, or use&& with a string literal:

assert(("There are five lights",2+2==5));assert(2+2==5&&"There are five lights");

The implementation ofassert inMicrosoft CRT does not conform to C99 and later revisions, because its underlying function (_wassert) takes neither__func__ nor an equivalent replacement.

Even though the change ofassert in C23 (N2829) is not a formal defect report, the C committeerecommends implementations to backport the change to old modes.

[edit]Example

#include <stdio.h>// uncomment to disable assert()// #define NDEBUG#include <assert.h>#include <math.h> #define TEST(...) __VA_ARGS__ int main(void){double x=-1.0;    assert(x>=0.0);printf("sqrt(x) = %f\n",sqrt(x));     assert(TEST(x>=0.0)); return0;}

--- Output with NDEBUG not defined: ---a.out: main.cpp:10: main: Assertion `x >= 0.0' failed. --- Output with NDEBUG defined: ---sqrt(x) = -nan

[edit]References

[edit]See also

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