Passing a template which requires a comma to a single-argument macro

Question

I have some code that essentially condenses down to

#define FOO(a)
FOO(std::map);

But it emits a compile error (too many ac

Answer 1

The preprocessor will only treat unparenthesised commas as a macro argument separator. So what you can do is rewrite std::map<int, int> into something that has parentheses around it. A simple one might be decltype(std::map<int, int>()).

Answer 2

This should perhaps ideally be a comment, but SO doesn't support code in comments, so, you can do

#include <map>

#define T_ARGS( ... ) < __VA_ARGS__ >

#define FOO( a )  a x;

auto main() -> int
{
    FOO( std::map T_ARGS( int, int ) );
    (void) x;
}

or you can define a macro that resolves to comma, or you can use just about any scheme that's specific to some particular use case (e.g., passing template name separately).

Answer 3

Just add an extra set of parentheses:

#define FOO(a)
FOO((std::map<int, int>));

Answer 4

The comma is being treated as a macro argument seperator, it does not do this with commas within parenthesizes.

If you are using Boost, they provide BOOST_PP_COMMA:

#include <boost/preprocessor/punctuation/comma.hpp>

#define FOO(a)
FOO(std::map<int BOOST_PP_COMMA int>);

You can also define your own:

#define COMMA ,
FOO(std::map<int COMMA int>);