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

Comma in C/C Macro: A Solution for Multiple Macro Arguments

Macros in C/C provide a convenient way to expand a preprocessor directive into a block of code. However, when macros accept multiple arguments, handling commas can become a challenge.

Consider the example macro:

#define FOO(type,name) type name

This macro expects two arguments: a type and a name. Using it is straightforward in simple cases:

FOO(int, int_var);

But when the type is more complex, such as a template or a container, passing commas can lead to errors:

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

One solution is to define a type alias to remove the need for commas:

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

However, this approach can be cumbersome and introduce type compatibility issues.

An alternative solution is to use a comma macro to handle the commas within the macro definition itself:

#define COMMA ,

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

This defines a comma macro that can be placed between arguments to handle the comma separation. This method provides a cleaner and more straightforward way to pass complex types to a macro:

#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 the type information for std::map:

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

Using a comma macro allows for more flexibility and cleaner syntax when working with macros that accept multiple arguments.

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