NAME¶

std::has_facet - std::has_facet

Synopsis¶

Defined in header <locale>
template< class Facet > (until C++11)
bool has_facet( const locale& loc ) throw();
template< class Facet > (since C++11)
bool has_facet( const locale& loc ) noexcept;

Checks if the locale loc implements the facet Facet.

The program is ill-formed if Facet is not a facet or it is a volatile-qualified
facet.

Parameters¶

loc - the locale object to query

Return value¶

Returns true if the facet Facet was installed in the locale loc, false otherwise.

Notes¶

std::has_facet must return true for all locales loc if Facet is one of the standard
facets given here.

Example¶

// Run this code

#include <iostream>
#include <locale>

// minimal custom facet
struct myfacet : public std::locale::facet
{
static std::locale::id id;
};

std::locale::id myfacet::id;

int main()
{
// loc is a "C" locale with myfacet added
std::locale loc(std::locale::classic(), new myfacet);
std::cout << std::boolalpha
<< "Can loc classify chars? "
<< std::has_facet<std::ctype<char>>(loc) << '\n'
<< "Can loc classify char32_t? "
<< std::has_facet<std::ctype<char32_t>>(loc) << '\n'
<< "Does loc implement myfacet? "
<< std::has_facet<myfacet>(loc) << '\n';
}

Output:¶

Can loc classify chars? true
Can loc classify char32_t? false
Does loc implement myfacet? true

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 436 C++98 it was unclear whether Facet it can be const-qualified, but not
can be cv-qualified volatile-qualified

See also¶

locale set of polymorphic facets that encapsulate cultural differences
(class)
use_facet obtains a facet from a locale
(function template)