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. Additionally, assert.h is available in C++.

Use

The assert macro implements runtime assertion. If the expression within fails, the assert macro will print a message to stderr and call the abort function. The message printed includes the source filename and the source line number, defined as the string literal __FILE__ and decimal constant __LINE__, respectively. Since C99, the message additionally includes the source function, of the identifier __func__, and the expression itself. In ANSI C, the expression in the assert macro must be a signed integer; in C99, the expression may be of any scalar type. Two common uses of the assert macro are to assert that a pointer is not null, otherwise known as a null pointer, and to ensure that indexes and size values are not negative and below a specified limit.

The value 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)

The assert macro does not include an error message. However, a custom message can be provided using the comma operator, as seen below.

assert(("Orwellian statement", 2 + 2 == 5));

Below, an example of a program using the assert macro is provided. The provided code will compile, as the value of the signed integer a is less than six.

#include <assert.h>

int main()
{
    int a = 5;
    assert(a < 6);

    return 0;
}

static_assert

The static_assert macro, added in C++11, serves a similar purpose to the assert macro. The static_assert macro 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 macro has no effect. In C++17, this assertion failure message was made optional, and the subsequent message is omitted if not specified. Alternatively, the <cassert> header from the assert.h header may also be used to declare the assert macro. assert.h may also be included directly to import the assert macro. In both cases, the assert macro in C++ is functionally equivalent to its C counterpart.

In C11, the _Static_assert declaration was added. Unlike the assert macro, _Static_assert is compile-time. As a result, _Static_assert results in a compilation error if its first argument evaluates to zero, or is false. assert.h defines static_assert as an alias for _Static_assert to ensure parity with C++. In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional. Gnulib defines static_assert for platforms that do not use C11 and does not require assert.h to be included.

References

Bibliography

• Ballman, Aaron; Grammatech (July 6, 2018). Harmonizing static_assert with C++ (Report).
• Binder, Robert (2000). Testing Object-oriented Systems: Models, Patterns, and Tools (2nd ed.). Boston: Addison-Wesley. ISBN 9780201809381.
• Gregoire, Marc (2021). Professional C++ (5th ed.). Hoboken: Wiley. ISBN 9781119695455.
• Gustedt, Jens (February 15, 2022). Revise spelling of keywords (Report).
• Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (2nd ed.). Hoboken: Prentice Hall. ISBN 9780131103627.
• Lischner, Ray (2009). C++ In a Nutshell: A Desktop Quick Reference (2nd ed.). Sebastopol: O'Reilly Media. ISBN 9781449378837.
• ISO/IEC JTC 1/SC 22/WG14 (December 1999). ISO/IEC 9899:1999 (Report).{{cite report}}: CS1 maint: numeric names: authors list (link)
• ISO/IEC JTC 1/SC 22/WG21 (January 2012). ISO/IEC 14882:2011 (Report).{{cite report}}: CS1 maint: numeric names: authors list (link)
• Prata, Stephen (2013). C Primer Plus (6th ed.). London: Pearson Education. ISBN 9780133432381.
• Swaminathan, Jeganathan (2017). Mastering C++ Programming. Birmingham: Packt. ISBN 9781786461629.