generic_selection.md

title	description	ms.date	helpviewer_keywords
Generic selection (C11)	Describes the C11 _Generic keyword used in the Microsoft Visual C compiler	12/9/2020	_Generic keyword [C]

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, except that 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 is not 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-list can't specify the same type more than once.
• The assoc-list can'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
*/

See also

/std (Specify Language Standard Version)
Type-generic math