The sizeof builtin can take either a type or an expression, and will return the appropriate value.
I'd like to build a macro that uses _Generic() expressions to do something similar. In particular, I'd like to be able to pass either a type name or a value as the parameter, just like sizeof.
As a trivial example, I can do something like this:
#define F(x) _Generic((x)+0, int: 1, long: 2)
That works because (x)+0 can be either an arithmetic expression or a type cast.
But that doesn't work when the type is struct Foo.
I could use composite literal syntax, with curly braces: (x){0} but that fails for literal values:
Is there some twisted C syntax horror that will permit the use of either type names or expressions in a _Generic-expression?
Your compound literal idea will work if you combine it with typeof.
However, please note that typeof is a a GCC C language extension, so it might not work in every C compiler.
#include <stdio.h>
typedef struct Foo {
int mx;
} Foo;
#define F(x) _Generic((typeof(x)){0}, \
int: 1, \
long: 2, \
struct Foo: 3)
int main()
{
printf("%d\n", F(0)); // 1
printf("%d\n", F(int)); // 1
printf("%d\n", F((long)0)); // 2
printf("%d\n", F(long)); // 2
printf("%d\n", F((Foo){1})); // 3
printf("%d\n", F(Foo)); // 3
}