c11 _Generic添加类型

Question

How do you add extra types to c11 _Generic Functions?

Do you have to #undef/re-#define it?(if so would the following work) or is there a nicer way?

#define to_str(X) _Generic((X), \
    long double: ld_str, \
    double: d_str, \
    float: f_str, \
    )(X)

#undef to_str

#define to_str(X) _Generic((X), \
    long double: ld_str, \
    double: d_str, \
    float: f_str, \
    int: i_str, \
    )(X)

Answer 1

I am not sure that I understand your question completely. You mean that you have a type generic macro that is given by some library and you want to amend it with a new type of your own?

What you always could do is to give it another name and use the default case to obtain the provided behavior:

#define to_str2(X) _Generic((X), default: to_str(X), int: i_str(X))

Edit :

This will not work perfectly because you'd have to put the function argument evaluation inside the _Generic . This means in particular that the type of X has to be compatible with all branches of the nested generic expressions.

It would be easier if the library in question had a macro that would just return the function itself, without the (X) , say to_strGen , and that never would evaluate X . Then you could do

#define to_str2Gen(X) _Generic((X), default: to_strGen(X), int: i_str)
#define to_str2(X) to_str2Gen(X)(X)

Answer 2

If it's your code, you would have to #undef it and re #define it, yes. There's no way to extend a type-generic expression (AFAIK).

If it's not your code I'd introduce a second expression with the extension like Jens suggested.