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Showing content from https://en.cppreference.com/w/cpp/language/../keyword/../preprocessor/../error/assert.html below:

assert - cppreference.com

Disabled assertion

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

#define assert(...)       ((void)0)

Enabled assertion

#define assert(condition) /* unspecified */

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

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

If

is defined as a macro name at the point in the source code where

or

is included, the assertion is disabled:

does nothing.

Otherwise, the assertion is enabled:

assert checks if its argument (which must have scalar type):

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

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().

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

• the text of condition

• #__VA_ARGS__

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

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:

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.

#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

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

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