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]

Use

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>
int main()
{
    void* pointer = 0;
    assert(pointer);
    return 0;
}

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

program: source.c:5: main: Assertion 'pointer' 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]

static_assert

The static_assert macro, 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.^{[citation needed]} 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.^[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.^[11][12] Gnulib defines static_assert for platforms that do not use C11 and does not require assert.h to be included.^[13]

References

Citations

Bibliography

• American National Standards Institute (1990). Rationale for the ANSI C Programming Language. Summit: Silicon Press. ISBN 9780929306070.
• 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.

This page was last edited on 9 June 2023, at 21:51