assert - cppreference.com (original) (raw)

| Defined in header | | | | ------------------------------------------------------------------------- | | ------------- | | 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

or

<assert.h>

is included, the assertion is disabled: assert does nothing.

  1. 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(). (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)

[edit] Parameters

condition - expression of scalar type

[edit] 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: (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.

[edit] Example

#include // uncomment to disable assert() // #define NDEBUG #include   // 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

[edit] 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

[edit] See also