Is there a way to both check a macro is defined and it equals a certain value at the same time

Question

I regularly use object-like preprocessor macros as boolean flags in C code to turn on and off sections of code.

For example

#define DEBUG_PRINT 1 

And then use it like

#if(DEBUG_PRINT == 1)     printf("%s", "Testing"); #endif 

However, it comes a problem if the header file that contains the #define is forgotten to be included in the source code. Since the macro is not declared, the preprocessor treats it as if it equals 0, and the #if statement never runs.

When the header file is forgotten to be included, non-expected, unruly behaviour can occur.

Ideally, I would like to be able to both check that a macro is defined, and check that it equals a certain value, in one line. If it is not defined, the preprocessor throws an error (or warning).

I'm looking for something along the lines of:

#if-def-and-true-else-throw-error(DEBUG_PRINT)     ... #endif 

It's like a combination of #ifdef and #if, and if it doesn't exist, uses #error.

I have explored a few avenues, however, preprocessor directives can't be used inside a #define block, and as far as I can tell, there is no preprocessor option to throw errors/warnings if a macro is not defined when used inside a #if statement.

Answer 1

This may not work for the general case (I don't think there's a general solution to what you're asking for), but for your specific example you might consider changing this sequence of code:

#if(DEBUG_PRINT == 1)     printf("%s", "Testing"); #endif 

to:

if (DEBUG_PRINT == 1) {     printf("%s", "Testing"); } 

It's no more verbose and will fail to compile if DEBUG_PRINT is not defined or if it's defined to be something that cannot be compared with 1.

Answer 2

Rather than using DEBUG_PRINT directly in your source files, put this in the header file:

#if !defined(DEBUG_PRINT)     #error DEBUG_PRINT is not defined #endif  #if DEBUG_PRINT     #define PrintDebug([args]) [definition] #else     #define PrintDebug #endif 

Any source file that uses PrintDebug but doesn't include the header file will fail to compile.

If you need other code than calls to PrintDebug to be compiled based on DEBUG_PRINT, consider using Michael Burr's suggestion of using plain if rather than #if (yes, the optimizer will not generate code within a false constant test).

Edit: And you can generalize PrintDebug above to include or exclude arbitrary code as long as you don't have commas that look like macro arguments:

#if !defined(IF_DEBUG)     #error IF_DEBUG is not defined #endif  #if IF_DEBUG     #define IfDebug(code) code #else     #define IfDebug(code) #endif 

Then you can write stuff like

IfDebug(int count1;)  // IfDebug(int count1, count2;) won't work IfDebug(int count2;) ... IfDebug(count1++; count2++;)