|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Defined in header <cassert> | ||
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 macroassert depends on another macro,NDEBUG, which is not defined by the standard library.
assert does nothing.
| (until C++26) |
| (since C++26) |
The diagnostic information has an implementation-defined format, but it always includes the following information:
| (until C++26) |
| (since C++26) |
The expressionassert(E) is guaranteed to be aconstant subexpression, if either
| (since C++11) |
Contents |
| condition | - | expression of scalar type |
Because assert(std::is_same_v<int,int>);// error: assert does not take two argumentsassert((std::is_same_v<int,int>));// OK: one argumentstatic_assert(std::is_same_v<int,int>);// OK: not a macro std::complex<double> c;assert(c==std::complex<double>{0,0});// errorassert((c==std::complex<double>{0,0}));// OK | (until C++26) |
There is no standardized interface to add an additional message toassert errors. A portable way to include one is to use acomma operator provided it has not beenoverloaded, or use&& with a string literal:
assert(("There are five lights",2+2==5));assert(2+2==5&&"There are five lights");
The implementation ofassert inMicrosoft 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 fromstd::source_location::current().
Even though the change ofassert in C23/C++26 is not formally a defect report, the C committeerecommends 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 failsstd::cout<<"Execution continues past the last assert\n";// No output}
Possible output:
Checkpoint #1Checkpoint #2Checkpoint #3Checkpoint #4main.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.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2234 | C++11 | assert could not be used in constant expression | can be used |
contract_assert statement(C++26) | verifies an internal condition during execution[edit] |
static_assert declaration(C++11) | performs compile-time assertion checking[edit] |
| causes abnormal program termination (without cleaning up) (function)[edit] | |
C documentation forassert | |
