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cgenericsgeneric-programmingc11

_Generic: multiple types to a single value?

Using c11 _Generic is there a way to map multiple types to a single value?

eg:

_Generic(e,      \
    A *:    foo_expression, \
    B **:   foo_expression, \
    C:      foo_expression, \
    enum D: bar_expression, \
    void *: bar_expression)

Is there a way to group types? (this isn't valid syntax just to express the intent)

_Generic(e, \
    A *: B **: C:    foo_expression, \
    enum D: void *:  bar_expression)

The reason I ask is args foo and baz can end up being large expressions (which can't necessarily be refactored into functions), so a way to avoid a lot of duplication would be good.

Note:

If there is no supported way using _Generic, I _could_ make a varargs macro to glue multiple args to a single value...

#define GENERIC_TYPE_GLUE3(answer, arg0, arg1) \
    arg0: answer, arg1: answer
#define GENERIC_TYPE_GLUE4(answer, arg0, arg1, arg2) \
    arg0: answer, arg1: answer, arg2: answer
....

... have many of these macros, then use a varargs wrapper to automatically use the right one, see: https://stackoverflow.com/a/24837037/432509

_Generic(e,      \
    GENERIC_TYPE_GLUE(foo_expression, short int, long),
    GENERIC_TYPE_GLUE(bar_expression, float, double))

(See working example: https://gist.github.com/ideasman42/4426f255880ff6a53080)

Solution

  • No, similar to switch there is no syntax foreseen to regroup multiple cases for _Generic. Your regrouping macro approach is probably the correct one for your purpose, then. You may have a look into macro meta programming as is done by boost or my package P99 that allow you to have just one such macro that also does the counting of the cases for.