Assert.h

<assert.h> is a header file in the C standard library. It defines the C preprocessor macro assert and implements runtime assertion in C.

<assert.h> is defined in ANSI C as part of the C standard library. In the C++ programming language, <assert.h> and <cassert> are available; both are functionally equivalent.¹

The assert macro implements runtime assertion. If the expression within it is false, the macro will print a message to stderr and call abort(), defined in <stdlib.h>. The message includes the source filename and the source line number from the macros __FILE__ and __LINE__, respectively.² Since C99, the name of the function the assert statement is included as (__FUNC__) and the expression itself.³ In ANSI C, the expression in the assert macro is defined as signed integer, although any expression that can be implicitly cast to a signed integer may be used. In C99, the assert macro explicitly allows any scalar type.⁴ Two common uses of the assert macro are to assert that a pointer is not null and to ensure that an array index is in-bounds.⁵

Below is a program using the assert macro. This program will always evaluate pointer as false, as pointer is a null pointer and does not point to a valid memory location:

#include <assert.h>
#include <stddef.h>

int main() {
    void* ptr = NULL;
    assert(ptr);
    return 0;
}

Upon compiling the program and running it, a message similar to the following will be output:

program: source.c:5: main: Assertion 'ptr' failed.
Aborted (core dumped)

The definition of the assert macro changes depending on the definition of another macro, NDEBUG. If NDEBUG is defined as a macro name, the assert macro is defined as #define assert(ignore) ((void)0),³ thus resulting in the macro not evaluating the expression. The use of NDEBUG may affect the overall behavior of a program if one or more assert statements contain side effects, as these statements are not evaluated.⁶

The assert macro does not include an error message. However the comma operator can be used to add it to the printed expression, as in assert(("Not Orwellian", 2 + 2 == 5));.⁷

The static_assert keyword, added in C++11, serves a similar purpose to the assert macro. Unlike the assert macro, static_assert runs at compile-time rather than at runtime.⁸ The original implementation used template hacks. The static_assert keyword takes in a constant expression that can be converted into a Boolean and a string literal; if the expression fails, the string literal is returned, otherwise, the assertion has no effect.⁸ In C++17, this assertion failure message was made optional, and the subsequent message is omitted if not specified.⁹

In C11, the functionally equivalent declaration _Static_assert was added. <assert.h> defines static_assert as an alias for _Static_assert to ensure parity with C++.¹⁰ In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional.¹¹¹² Gnulib defines static_assert for platforms that do not use C11 and does not require <assert.h> to be included.¹³

The contract_assert keyword, added in C++26, is for contract assertions and used to verify internal conditions similar to the assert() macro by ensuring that a condition holds during execution.¹⁴¹⁵

int f(vector<int>& v)
    pre (v.size() >= 1 && v[0] > 0)
    post (r: r == v[0] && r != 1) {
    // ...
    contract_assert(v[0] != 1);
    // ...
    return v[0];
}

In Java, assert is a keyword.

In C#, there is no assertion macro or keyword, but instead classes System.Diagnostics.Debug and System.Diagnostics.Trace which provide Assert() methods.

In Rust, there is an assert!() macro.