std::has_facet(3)

💅 std::has_facet — the C++ Standard Library glow-up

okay bestie, so this one is from the C++ Standard Library manual set (std::has_facet(3)), and it's genuinely one of those little functions that sounds scary and is actually just a vibe check for locales. let's get into it 🫶

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✨ NAME ✨

std::has_facet — literally just std::has_facet, no notes, the name is the name 💅

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🌸 SYNOPSIS 🌸

so <locale> is the header it lives in, and it's got two eras, ok? pre-C++11 and post-C++11, and the only glow-up is throw() becoming noexcept:

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;

what it does: checks if the locale loc implements the facet Facet 🦋

and bestie, listen to me, this is the important bit: the program is ill-formed if Facet is not a facet, or if it's a volatile-qualified facet. not "oops undefined behaviour" ill-formed, like, ill-formed. no cap. 😭

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🎀 Parameters 🎀

• loc — the locale object you're querying, the one you're side-eyeing to see what it's got 👀

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⭐ Return value ⭐

returns true if the facet Facet was installed in the locale loc, and false otherwise. that's it. that's the whole function. slay. ✨

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💜 Notes 💜

std::has_facet must return true for all locales loc if Facet is one of the standard facets given here (iykyk, check the standard list 💅).

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🌈 Example 🌈

run this code, it's adorable:

#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

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🧚 Defect reports 🧚

so these are behaviour-changing defect reports that got applied retroactively to previously published C++ standards. history really said "let's fix that" 💀

• DR LWG 436, applied to C++98 — the behaviour as published was that it was unclear whether Facet can be cv-qualified. the correct behaviour: it can be const-qualified, but not volatile-qualified 👑

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👑 See also 👑

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

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kk that's the whole page, you ate it, no crumbs 💖 this was served fresh from cppreference.com, dated 2024.06.10 — go be the main character of your own locale, bestie 🦋✨🎀