[assertions]

If NDEBUG is defined as a macro name at the point in the source file where <cassert> is included, the assert macro is defined as #define assert(.) ((void)0)

Otherwise, the assert macro puts a diagnostic test into programs; it expands to an expression of type void which has the following effects:

(2.1)
__VA_ARGS__ is evaluated and contextually converted to bool.
(2.2)
If the evaluation yields true there are no further effects.
(2.3)
Otherwise, the assert macro's expression creates a diagnostic on the standard error stream (ISO/IEC 9899:2024, 7.23.3) in an implementation-defined format and calls abort().

The diagnostic contains #__VA_ARGS__ and information on the name of the source file, the source line number, and the name of the enclosing function (such as provided by source_location::current()).

If __VA_ARGS__ does not expand to an assignment-expression, the program is ill-formed.

The macro assert is redefined according to the current state of NDEBUG each time that <cassert> is included.

An expression assert(E) is a constant subexpression, if

(5.1)
NDEBUG is defined at the point where assert is last defined or redefined, or
(5.2)
E contextually converted to bool is a constant subexpression that evaluates to the value true.

19.3.2 Header <cassert> synopsis [cassert.syn]

19.3.3 The assert macro [assertions.assert]