# assert

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Defined in header `<cassert>` |  |
#ifdef NDEBUG #define assert(condition) ((void)0) #else #define assert(condition) /*implementation defined*/ #endif |  |

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 `<cassert>` is included, then `assert` does nothing.

If NDEBUG is not defined, then `assert` checks if its argument (which must have scalar type) compares equal to zero. If it does, `assert` outputs implementation-specific diagnostic information on the standard error output and calls std::abort. The diagnostic information is required to include the text of `expression`, as well as the values of the standard macros __FILE__, __LINE__, and the standard variable __func__.

The expression `assert(E)` is guaranteed to be a constant subexpression, if either

- NDEBUG is defined at the point where `assert` is last defined or redefined (i.e., where the header `<cassert>` or `<assert.h>` was last included); or
- `E`, contextually converted to bool, is a constant subexpression that evaluates to true.

(since C++17)

### Parameters

condition | - | expression of scalar type

### Return value

(none)

### Notes

Because `assert` is a function-like macro, commas anywhere in condition that are not protected by parentheses are interpreted as macro argument separators. Such commas are often found in template argument lists:

    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

### Example

Run this code

    #include <iostream>
    // uncomment to disable assert()
    // #define NDEBUG
    #include <cassert>

    int main()
    {
        assert(2+2==4);
        std::cout << "Execution continues past the first assert\n";
        assert(2+2==5);
        std::cout << "Execution continues past the second assert\n";
    }

Possible output:

    Execution continues past the first assert
    test: test.cc:10: int main(): Assertion `2+2==5' failed.
    Aborted

### See also

static assertion | performs compile-time assertion checking (since C++11)
abort | causes abnormal program termination (without cleaning up) (function)
C documentation for assert

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