assert

 

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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): If the argument compares equal to zero, there are no further effects. Otherwise, assert creates a diagnostic on the standard error stream and calls std::abort().	(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, assert creates a diagnostic on the standard error stream and calls std::abort().	(since C++26)

The diagnostic information has an implementation-defined format, but it always includes the following information:

the text of condition	(until C++26)
#__VA_ARGS__	(since C++26)

• the source file name (i.e., __FILE__)
• the source line number (i.e., __LINE__)
• the name of the enclosing function (i.e., __func__)

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++11)

Parameters

condition

-

expression of scalar type

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.

Since C++20, the values needed for the diagnostic message can also be obtained from std::source_location::current().

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

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, "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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2234	C++11	assert could not be used in constant expression	can be used

See also

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