assert

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

Parameters

Return value

(none)

Notes

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.

Even though the change of assert in C23/C++26 is not formally a defect report, the C committee recommends implementations to backport the change to old modes.

Example

#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, "Success");
    std::cout << "Checkpoint #4\n";
 
    assertm(2 + 2 == 5, "Failed"); // assertion fails
    std::cout << "Execution continues past the last assert\n"; // No output
}

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

See also