assert.h is a header file in the standard library of the C programming language that defines the C preprocessor macro assert(). In C++ it is also available through the <cassert> header file. Assert assert(a != 1); This is a macro that implements a runtime assertion, which can be used to verify assumptions made by the program and print a diagnostic message if this assumption is false. When executed, if the expression is false (that is, compares equal to 0), assert() will write information about the call that failed on stderr and then call abort(). The information it writes to stderr includes: the source filename (the predefined macro ) the source line number (the predefined macro ) the source function (the predefined identifier __func__) (added in C99) the text of expression that evaluated to 0 Example output of a program compiled on Linux: program: program.c:5: main: Assertion `a != 1' failed. Abort (core dumped) Programmers can eliminate the assertions just by recompiling the program, without changing the source code: if the macro NDEBUG is defined before the inclusion of <assert.h>, the assert() macro may be defined simply as: #define assert(ignore)((void) 0) and therefore has no effect on the compilation unit, not even evaluating its argument. Therefore expressions passed to assert() must not contain side-effects since they will not happen when debugging is disabled. For instance: assert(x = gets()); will not read a line and not assign to x when debugging is disabled. Additional message There is no standardized variant of assert() that includes an error message. This can nevertheless be achieved using a comma operator, which discards all preceding values and only keeps the last one: assert(("Orwellian", 2 + 2 == 5)); Will yield something similar to: program: program.c:5: main: Assertion `("Orwellian", 2 + 2 == 5)' failed. Abort A more convenient syntax can be made with a macro, here using the name Microsoft uses for a similar macro: #define _ASSERT_EXPR(test, message) assert(((void)(message), test)) _ASSERT_EXPR(2 + 2 == 5, "Orwellian"); Static assert static_assert(sizeof(int) > 20, "I need huge integers"); C++11 added a similar keyword static_assert that contextually converts a constant expression to bool and prints a message (optional since C++17) at compile-time if it is false. It is possible to simulate this using a macro and templates, though this is probably not necessary since most modern C++ compilers support this C++11 feature. This feature was formally added in C11 as the