C11 introduces the ability to select different expressions based on the type of an expression, using a generic selection.

primary-expression

generic-selection

since C11

generic-selection

_Generic ( assignment-expression , generic-assoc-list )

generic-assoc-list

generic-association

generic-assoc-list , generic-association

generic-association

type-name : assignment-expression

default : assignment-expression

For example:

const char *y = _Generic(v,
                         int    : "%d",
                         long   : "%ld",
                         double : "%g");

Depending on the type of the controlling expression v, which is not evaluated, one of the three strings %d, %ld and %g is chosen as the value of the generic expression. It is an error if the type is not one of those listed, unless one of the options is identified with default:

const char *y = _Generic(v,
                         int     : "%d",
                         default : "%ld",
                         double  : "%g");

Of course, the type of v is known at translation time, so the selection is made and fixed then. Furthermore, even the programmer might know what type v has before translation starts, so this kind of expression might seem redundant in this context. Two cases where it wouldn't be redundant are when:

• The type of the controlling expression is prone to change outside the programmer's control, especially if it is identified with a type alias.

• The selection is within a macro definition, and the controlling expression is derived from a macro argument, so the type is not known until the macro is expanded. Macros defined in <tgmath.h> are likely to be covered by this case.

One issue with _Generic is that no two options may identify the same type. So, if size_t is unsigned long, the expression _Generic(cx, size_t : x1, unsigned long : x2) would fail, yet it would be okay on a system where size_t is unsigned long long. One solution to this might be to cascade several generic selections:

_Generic(cx,
         size_t : x1,
         default : _Generic(cx,
                            unsigned long : x2))