# std::has_facet

From cppreference.com
< cpp‎ | locale

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

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

### See also

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/locale/has_facet&oldid=148215"