How Can I Handle Commas Correctly in C/C Macros with Multiple Arguments?

Macros and Commas in C/C

In C/C , macros are often used to simplify and generalize code. However, when macros take multiple arguments, it can be difficult to handle commas correctly.

Consider the following macro:

#define FOO(type, name) type name

This macro can be used to declare a variable of a specific type, for example:

FOO(int, int_var);

However, if the type contains commas, such as in a template template parameter, the macro fails. For instance:

FOO(std::map<int, int>, map_var); // error: macro "FOO" passed 3 arguments, but takes just 2

To resolve this issue, one option is to introduce a typedef:

typedef std::map<int, int> map_int_int_t;
FOO(map_int_int_t, map_var); // OK

While this approach works, it can be cumbersome and introduce type compatibility issues.

An alternative solution is to use the preprocessor's COMMA macro:

#define COMMA ,

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

This allows commas to be included in macro arguments without causing errors. Additionally, it can be used for splicing macro arguments into strings:

#include <cstdio>
#include <map>
#include <typeinfo>

#define STRV(...) #__VA_ARGS__
#define COMMA ,
#define FOO(type, bar) bar(STRV(type) \
    " has typeid name \"%s\"", typeid(type).name())

int main()
{
    FOO(std::map<int COMMA int>, std::printf);
}

This code prints:

std::map<int , int> has typeid name "St3mapIiiSt4lessIiESaISt4pairIKiiEEE"

By utilizing the COMMA macro, developers can avoid typedefs and ensure that commas are handled correctly in macros.

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