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std::has_facet

Defined in header `<locale>`
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)

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

#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` can be cv-qualified	it can be const-qualified, but not volatile-qualified