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std::codecvt<InternT,ExternT,StateT>::always_noconv, do_always_noconv - cppreference.com

From cppreference.com

Defined in header <locale>

(1)

public: bool always_noconv() const throw();

 (until C++11)

public: bool always_noconv() const noexcept;

 (since C++11)
(2)

protected: virtual bool do_always_noconv() const throw();

 (until C++11)

protected: virtual bool do_always_noconv() const noexcept;

 (since C++11)

1) Public member function, calls the member function do_always_noconv of the most derived class.

2) Returns true if both do_in() and do_out() return std::codecvt_base::noconv for all valid inputs.

Return value

true if this conversion facet performs no conversions, false otherwise.

The non-converting specialization std::codecvt<char, char, std::mbstate_t> returns true.

Notes

This function may be used e.g. in the implementation of std::basic_filebuf::underflow and std::basic_filebuf::overflow to use bulk character copy instead of calling std::codecvt::in or std::codecvt::out if it is known that the locale imbued in the std::basic_filebuf does not perform any conversions.

Example

#include <iostream>
#include <locale>

int main()
{
    std::cout << "The non-converting char<->char codecvt::always_noconv() returns " 
              << std::boolalpha
              << std::use_facet<std::codecvt<char, char, std::mbstate_t>>(
                    std::locale()
                 ).always_noconv() << '\n'
              << "while wchar_t<->char codecvt::always_noconv() returns "
              << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                    std::locale()
                 ).always_noconv() << '\n';
}

Output: 

The non-converting char<->char codecvt::always_noconv() returns true
while wchar_t<->char codecvt::always_noconv() returns false