Can I conditionally replace preprocessor arguments?

Question

Working with a unit test framework, I came across a situation in which I'd like to test macro arguments. Simply said, I'd like to expand the macro FOO(x) such that FOO(int) would be short and FOO(anything_else) would be long.

With C++ templates, of course this isn't a problem. But here I need a real token replacement, not just a typedef. I.e. FOO(char) FOO(char) i; should be a valid definition equal to long long i;.

The newest C standard has feature where the replacement text of a macro can depend on the expression type of the first argument. You could use this as a building block. — Stefan Majewsky, Jun 19 '12 at 14:45
Did you have a look at boost.pp and if you can hack somethign together with it? — PlasmaHH, Jun 19 '12 at 15:09

score 3 · Answer 1 · edited May 23 '17 at 12:07

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As far as I know, the only string-like operations available in C macros are pasting/concatenating tokens (using ##), and string-izing them (using #).

I'm pretty sure the closest you're going to get involves enumerating the possibilities like so:

#define FOO(x) FOO__##x
#define FOO__int   short
#define FOO__short long
#define FOO__long  long
#define FOO__char  long
// ... for each type you want to replace

Inspiration from this question.

edited May 23 '17 at 12:07

Community

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answered Jun 19 '12 at 16:43

vergenzt

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(Edit #2): if you have limited and known set of "elses", one could list them all. – flolo Jun 19 '12 at 17:07
Yeah, I realized that just now, and cleaned up the whole post. – vergenzt Jun 19 '12 at 17:07
I don't think I can enumerate my arguments up front; I might get a `FOO(AcmeWidgetUnitTest17)` :( – MSalters Jun 21 '12 at 08:51

score 0 · Answer 2 · answered Jun 19 '12 at 16:36

0

what you are trying to do is impossible.

Macros are evaluated by the c preprocessor, which as the name implies runs before the compiler runs. It doesn't know what the types of your symbols are yet.

Why don't you create a class for the type that casts itself to the right thing at the time it is evaluated by the compiler.

answered Jun 19 '12 at 16:36

Rafael Baptista

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I don't care about types. I know that `int` at the preprocessing stage is just a token. Still, I want to act conditionally on the token `int`. – MSalters Jun 21 '12 at 08:43

Can I conditionally replace preprocessor arguments?

2 Answers2