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

问题

I have some code that essentially condenses down to

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

But it emits a compile error (too many actual parameters for macro FOO).

Obviously the preprocessor is thinking that I've supplied std::map<int and int> as arguments.

Is there a way round this? The preprocessor will not treat a quoted string with a comma in this way.

回答1:

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).

回答2:

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>);

回答3:

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>()).

回答4:

Just add an extra set of parentheses:

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

来源：https://stackoverflow.com/questions/32460599/passing-a-template-which-requires-a-comma-to-a-single-argument-macro

标签

c++

c-preprocessor