Generic selection (C11)
Use the _Generic keyword to write code that selects an expression at compile time based on the type of the argument. It's similar to overloading in C++ where the type of the argument selects which function to call. In this case, the type of the argument selects which expression to evaluate.
For example, the expression _Generic(42, int: "integer", char: "character", default: "unknown"); evaluates the type of 42 and looks for the matching type, int, in the list. It finds it and returns "integer".
Syntax
generic-selection:
_Generic ( assignment-expression, assoc-list )
assoc-list:
association
assoc-list, association
association:
type-name : assignment-expression
default : assignment-expression
The first assignment-expression is called the controlling expression. The type of the controlling expression is determined at compile time and matched against the assoc-list to find which expression to evaluate and return. The controlling expression isn't evaluated. For example, _Generic(intFunc(), int: "integer", default: "error"); doesn't result in a call at runtime to intFunc.
When the type of the controlling expression is determined, const, volatile, and restrict are removed before matching against assoc-list.
Entries in the assoc-list that aren't chosen aren't evaluated.
Constraints
- The
assoc-listcan't specify the same type more than once. - The
assoc-listcan't specify types that are compatible with each other, such as an enumeration and the underlying type of that enumeration. - If a generic selection doesn't have a default, the controlling expression must have only one compatible type name in the generic association list.
Example
One way to use _Generic is in a macro. The <tgmath.h> header file uses _Generic to call the right math function depending on the type of argument. For example, the macro for cos maps a call with a float to cosf, while mapping a call with a complex double to ccos.
The following example shows how to write a macro that identifies the type of the argument you pass to it. It produces "unknown" if no entry in the assoc-list matches the controlling expression:
// Compile with /std:c11
#include <stdio.h>
/* Get a type name string for the argument x */
#define TYPE_NAME(X) _Generic((X), \
int: "int", \
char: "char", \
double: "double", \
default: "unknown")
int main()
{
printf("Type name: %s\n", TYPE_NAME(42.42));
// The following would result in a compile error because
// 42.4 is a double, doesn't match anything in the list,
// and there is no default.
// _Generic(42.4, int: "integer", char: "character"));
}
/* Output:
Type name: double
*/
Requirements
Compile with /std:c11.
Windows SDK 10.0.20348.0 (version 2104) or later. See Windows SDK to download the latest SDK. For instructions to install and use the SDK for C11 and C17 development, see Install C11 and C17 support in Visual Studio.