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Generic selection

From cppreference.com
< c‎ | language

Provides a way to choose one of several expressions at compile time, based on a type of a controlling expression

Contents

[edit] Syntax

_Generic ( controlling-expression , association-list ) (since C11)

where association-list is a comma-separated list of associations, each of which has the syntax

type-name : expression
default : expression

where

type-name - any complete object type that isn't variably-modified (that is, not VLA or pointer to VLA).
controlling-expression - any expression (except for the comma operator) whose type must be compatible with one of the type-names if the default association is not used
expression - any expression (except for the comma operator) of any type and value category

No two type-names in the association-list may specify compatible types. There may be only one association that uses the keyword default. If default is not used and none of the type-names are compatible with the type of the controlling expression, the program will not compile.

[edit] Explanation

First, the type of controlling-expression undergoes lvalue conversions. The conversion is performed in type domain only: it discards the top-level cvr-qualifiers and atomicity and applies array-to-pointer/function-to-pointer transformations to the type of the controlling expression, without initiating any side-effects or calculating any values.

The type after conversion is compared with type-names from the list of associations.

If the type is compatible with the type-name of one of the associations, then the type, value, and value category of the generic selection are the type, value, and value category of the expression that appears after the colon for that type-name.

If none of the type-names are compatible with the type of the controlling-expression, and the default association is provided, then the type, value, and value category of the generic selection are the type, value, and value category of the expression after the default : label.

[edit] Notes

The controlling-expression and the expressions of the selections that are not chosen are never evaluated.

Because of the lvalue conversions, "abc" matches char* and not char[4] and (int const){0} matches int, and not const int.

All value categories, including function designators and void expressions, are allowed as expressions in a generic selection, and if selected, the generic selection itself has the same value category.

The type-generic math macros from <tgmath.h>, introduced in C99, were implemented in compiler-specific manner. Generic selections, introduced in C11, gave the programmers the ability to write similar type-dependent code.

Generic selection is similar to overloading in C++ (where one of several functions is chosen at compile time based on the types of the arguments), except that it makes the selection between arbitrary expressions.

[edit] Keywords

_Generic, default

[edit] Example

#include <stdio.h>
#include <math.h>
 
// Possible implementation of the tgmath.h macro cbrt
#define cbrt(X) _Generic((X), \
              long double: cbrtl, \
                  default: cbrt,  \
                    float: cbrtf  \
)(X)
 
int main(void)
{
    double x = 8.0;
    const float y = 3.375;
    printf("cbrt(8.0) = %f\n", cbrt(x)); // selects the default cbrt
    printf("cbrtf(3.375) = %f\n", cbrt(y)); // converts const float to float,
                                            // then selects cbrtf
}

Output:

cbrt(8.0) = 2.000000
cbrtf(3.375) = 1.500000

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

DR Applied to Behavior as published Correct behavior
DR 481 C11 it was underspecified if the controlling expression undergoes lvalue conversions it undergoes

[edit] References

  • C17 standard (ISO/IEC 9899:2018):
  • 6.5.1.1 Generic selection (p: 56-57)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.5.1.1 Generic selection (p: 78-79)