Accessing variable values within a macro

Question

Some time ago, I made this beautiful assert macro for c and c++ programs

#define ASSERT(truthy, message) \
     if (!(truthy)) \
     {\
         cout << message << " on line " << __LINE__ << " in file " << __FILE__ << ". Check was " << #truthy << endl;\
     }

Scatter ASSERT calls throughout your code, and it will warn you whenever the truthy value is not truthy! Very handy during development to remind you of potential mistakes.

ex

ASSERT(filesFound > 0, "Couldn't find any files, check your path!");

When filesFound is 0, the macro will print out

Couldn't find any files, check your path! on line 27 in file openFiles.c. Check was filesFound > 0

Now what I want it to print, to give me even more relevant information, is the value of any variables passed into the truthy parameter. Like this

Couldn't find any files, check your path! on line 27 in file openFiles.c. Check was filesFound > 0, filesFound is 0

This seems lisp-like territory, I wonder, is there any black magic c preprocessing that I can use to evaluate variables and functions to their values, without evaluating the truthy statement?

I assume to be disappointed.

Answer 1

An alternative solution which I've always used is to support varargs in the macro and then force the assert user to specify the relevant message / variables - it's a little bit of extra work each time, but on the plus side you can get exactly the formatting that you want and include information not available in the "truthy" bit, e.g:

#define ASSERT(truthy, message, ...) \
if (!(truthy)) \
{\
    MyAssertHandler(__LINE__, __FILE__, #truthy, message, ##__VA_ARGS__);
}

Then you're handler is just a fairly standard var-arg function that can use e.g. vsnprintf to generate the message and output it, e.g. off the top of my head:

void MyAssertHandler(int line, const char* file, const char* expressionStr, const char* format, ...)
{
    // Note: You probably want to use vsnprintf instead to first generate
    //       the message and then add extra info (line, filename, etc.) to
    //       the actual output 
    va_list args;
    va_start(args, format);
    vprintf(format, args);
    va_end(args);

    // Log to bug database, DebugBreak() if a debugger is attached, etc.
}

usage:

ASSERT(IsBlah(), "BlahBlah: x = %.2f, name = %s", GetX(), GetName());