assert
(C++17)<cassert>
The definition of the macro assert depends on another macro, NDEBUG, which is not defined by the standard library.
assert does nothing.assert checks if its argument (which must have scalar type):
- If the argument compares unequal to zero, there are no further effects.
- Otherwise,
assertcreates 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,
assertcreates 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__ )
The expression assert(E) is guaranteed to be a constant subexpression, if either
- NDEBUG is defined at the point where
assertis last defined or redefined, or - E, contextually converted to bool, is a constant subexpression that evaluates to true.
[edit] Parameters
[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:
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
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 <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
[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 |