How to use #if inside #define in the C preprocessor?

Question

I want to write a macro that spits out code based on the boolean value of its parameter. So say DEF_CONST(true) should be expanded into const, and DEF_CONST(false) should be expanded into nothing.

Clearly the following doesn't work because we can't use another preprocessor inside #defines:

#define DEF_CONST(b_const) \ #if (b_const) \   const \ #endif 

Answer 1

You can simulate conditionals using macro token concatenation as follows:

#define DEF_CONST(b_const) DEF_CONST_##b_const #define DEF_CONST_true const #define DEF_CONST_false 

Then,

/* OK */ DEF_CONST(true)  int x;  /* expands to const int x */ DEF_CONST(false) int y;  /* expands to int y */  /* NOT OK */ bool bSomeBool = true;       // technically not C :) DEF_CONST(bSomeBool) int z;  /* error: preprocessor does not know the value                                 of bSomeBool */ 

Also, allowing for passing macro parameters to DEF_CONST itself (as correctly pointed out by GMan and others):

#define DEF_CONST2(b_const) DEF_CONST_##b_const #define DEF_CONST(b_const) DEF_CONST2(b_const) #define DEF_CONST_true const #define DEF_CONST_false  #define b true #define c false  /* OK */ DEF_CONST(b) int x;     /* expands to const int x */ DEF_CONST(c) int y;     /* expands to int y */ DEF_CONST(true) int z;  /* expands to const int z */ 

You may also consider the much simpler (though potentially less flexible):

#if b_const # define DEF_CONST const #else /*b_const*/ # define DEF_CONST #endif /*b_const*/ 

Answer 2

Doing it as a paramterised macro is a bit odd.

Why not just do something like this:

#ifdef USE_CONST     #define MYCONST const #else     #define MYCONST #endif 

Then you can write code like this:

MYCONST int x = 1; MYCONST char* foo = "bar"; 

and if you compile with USE_CONST defined (e.g. typically something -DUSE_CONST in the makefile or compiler options) then it will use the consts, otherwise it won't.

Edit: Actually I see Vlad covered that option at the end of his answer, so +1 for him :)