BYO assertion macros

BYO assertion macros

An important point about the default “assert” macro is that when the condition fails, the default C++ library crashes your program by calling the standard “abort” function, which triggers a fatal exception on Windows or a core dump on Linux. That is fine for debugging, but it isn't what you want for production code, so most professional C++ programmers declare their own assertion macros instead.

For example, here's my own “yassert” macro in a “yasssert.h” header file:

    #define yassert(cond) ( (cond) || aussie_yassert_fail(#cond, __FILE__, __LINE__) )

This tricky macro uses the short-circuiting of the “||” operator, which has a meaning like “or-else”. So, think of it this way: the condition is true, or else we call the failure function. The effect is similar to an if-else statement, but an expression is cleaner in a macro.

The __FILE__ and __LINE__ preprocessor macros expand to the current filename and line number. The filename is a string constant, whereas the line number is an integer constant.

Function names: Note that you can add “__func__” to also report the current function name if you wish. There's also an older non-standard __FUNCTION__ version of the macro. Note that the need for all these macros goes away once there is widespread C++ support for std::stacktrace, as standardized in C++23, in which case a failing assertion could simply report its own call stack in an error message.

When Assertions Fail. This yassert macro relies on a function that is called only when an assertion has failed. And the function has to have a dummy return type of “bool” so that it can be used as an operand of the || operator, whereas a “void” return type would give a compilation error. Hence, the declaration is:

    bool aussie_yassert_fail(char* str, char* fname, int ln);  // Assertion failed

And here's the definition of the function:

    bool aussie_yassert_fail(char* str, char* fname, int ln)  
    {
        // Assertion failure has occurred...
        g_aussie_assert_failure_count++;
        fprintf(stderr, "AUSSIE ASSERTION FAILURE: %s, %s:%d\n", str, fname, ln);
        return false;  // Always fails
    }

This assertion failure function must always return “false” so that the assertion macro can be used in an if-statement condition.

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