assert

Header: <cassert>

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

# Declarations

Disabled assertion

#define assert(condition) ((void)0)

(until C++26)

#define assert(...) ((void)0)

(since C++26)

Enabled assertion

#define assert(condition) /* unspecified */

(until C++26)

#define assert(...) /* unspecified */

(since C++26)

# 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:

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:

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

#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
}

# Defect reports

# See also

• static_assert declaration
• abort
• C documentation