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Defined in header <cassert>
#ifdef NDEBUG #  define assert(condition) ((void)0) #else #  define assert(condition) /*implementation defined*/ #endif

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

If NDEBUG is defined as a macro name at the point in the source code where <cassert> or <assert.h> is included, then assert does nothing.

If NDEBUG is not defined, then assert checks if its argument (which must have scalar type) compares equal to zero. If it does, assert outputs implementation-specific diagnostic information on the standard error output and calls std::abort. The diagnostic information is required to include the text of expression, as well as the values of the predefined variable __func__ and (since C++11) the predefined macros __FILE__ and __LINE__.

The expression assert(E) is guaranteed to be a constant subexpression, if either NDEBUG is defined at the point where assert is last defined or redefined (i.e., where the header <cassert> or <assert.h> was last included); or E, contextually converted to bool, is a constant subexpression that evaluates to true.

(since C++17)

Parameters

condition

-

expression of scalar type

Return value

(none)

Notes

Because assert is a function-like macro, commas anywhere in condition that are not protected by parentheses are interpreted as macro argument separators. Such commas are often found in template argument lists and list-initialization:

assert(std::is_same_v<int, int>); // error: assert does not take two arguments
assert((std::is_same_v<int, int>)); // OK: one argument
static_assert(std::is_same_v<int, int>); // OK: not a macro
std::complex<double> c;
assert(c == std::complex<double>{0, 0}); // error
assert((c == std::complex<double>{0, 0})); // OK

There is no standardized interface to add an additional message to assert errors. A portable way to include one is to use a comma operator provided it has not been overloaded, or use && with a string literal:

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

The implementation of assert in Microsoft CRT does not conform to C++11 and later revisions, because its underlying function (_wassert) takes neither __func__ nor an equivalent replacement.

Example

Run this code

#include <iostream>
// uncomment to disable assert()
// #define NDEBUG
#include <cassert>
 
// Use (void) to silence unused warnings.
#define assertm(exp, msg) assert(((void)msg, exp))
 
int main()
{
    assert(2+2==4);
    std::cout << "Checkpoint #1\n";
 
    assert((void("void helps to avoid 'unused value' warning"), 2*2==4));
    std::cout << "Checkpoint #2\n";
 
    assert((010+010==16) && "Yet another way to add an assert message");
    std::cout << "Checkpoint #3\n";
 
    assertm((2+2)%3==1, "Expect expected");
    std::cout << "Checkpoint #4\n";
 
    assertm(2+2==5, "There are five lights"); // assertion fails
    std::cout << "Execution continues past the last assert\n"; // No
}

Possible output:

Checkpoint #1
Checkpoint #2
Checkpoint #3
Checkpoint #4
main.cpp:23: int main(): Assertion `((void)"There are five lights", 2+2==5)' failed.
Aborted

See also

static_assert declaration(C++11)	performs compile-time assertion checking
abort	causes abnormal program termination (without cleaning up) (function)