c11 _Generic adding types

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)