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.
- If NDEBUG is defined as a macro name at the point in the source code where
or
is included, the assertion is disabled: assert 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(). | (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__)
[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 |