How Can I Handle Commas in Complex C/C Macro Arguments?

Comma in C/C Macros

In C/C macros, it is often necessary to separate multiple arguments with commas. However, issues arise when the macro expects a specific number of arguments, and providing more or less causes errors.

Problem Scenario

Consider a macro defined as follows:

#define FOO(type, name) type name

While this macro can be used simply, e.g.:

FOO(int, int_var);

It becomes problematic when complex types are involved:

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

Traditional Solution

One common approach is to create a new type alias and use it within the macro:

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

Macro-Based Solution

Alternatively, you can define a macro representing a comma:

#define COMMA ,

Using this macro, you can now pass complex types without errors:

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

Additional Use Case

This approach also proves useful when stringifying macro arguments, as illustrated below:

#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 will print:
std::map has typeid name "St3mapIiiSt4lessIiESaISt4pairIKiiEEE"

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