std::has_facet

Since C++11

Header: <locale>

Checks if the locale loc implements the facet Facet.

# Declarations

template< class Facet >
bool has_facet( const locale& loc ) throw();

(until C++11)

template< class Facet >
bool has_facet( const locale& loc ) noexcept;

(since C++11)

# 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

#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';
}

# Defect reports

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