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assert

Defined in header <cassert>
Disabled assertion
(1) 
#define assert(condition) ((void)0)
 (until C++26) 
#define assert(...)       ((void)0)
 (since C++26)
Enabled assertion
(2) 
#define assert(condition) /* unspecified */
 (until C++26) 
#define assert(...)       /* unspecified */
 (since C++26)

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

1) If NDEBUG is defined as a macro name at the point in the source code where <cassert> or <assert.h> is included, the assertion is disabled: assert does nothing.
2) Otherwise, the assertion is enabled:

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 condition, as well as the values of the predefined variable __func__ and(since C++11) the predefined macros __FILE__ and __LINE__.

 (until C++26)

assert puts a diagnostic test into programs and expands to an expression of type void. __VA_ARGS__ is evaluated and contextually converted to bool:

  • If the evaluation yields true, there are no further effects.
  • Otherwise, the assert macro’s expression creates a diagnostic on the standard error stream in an implementation-defined format and calls std::abort(). The diagnostic contains #__VA_ARGS__ and information on the the source file name, the source line number, and the name of the enclosing function (such as provided by std::source_location::current()).
 (since C++26)

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, 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 the argument 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
 (until C++26)

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

#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
}

Possible output:

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

See also

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

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