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std::as_const

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<tbody> </tbody>
template< class T > constexpr std::add_const_t<T>& as_const( T& t ) noexcept;
 (1) ( C++17)
template< class T > void as_const( const T&& ) = delete;
 (2) ( C++17)
1) lvalue t.
2) const rvalue , rvalue.

template <class T>
constexpr std::add_const_t<T>& as_const(T& t) noexcept
{
    return t;
}

__cpp_lib_as_const 201510L (C++17) std::as_const 

#include <cassert>
#include <string>
#include <type_traits>
#include <utility>

int main()
{
    std::string mutableString = " !";
    auto&& constRef = std::as_const(mutableString);

    mutableString.clear(); // OK
//  constRef.clear(); // : 'constRef'   'const',
                      //          'clear'    const

    assert( &constRef == &mutableString );
    assert( &std::as_const( mutableString ) == &mutableString );

    using ExprType = std::remove_reference_t<decltype(std::as_const(mutableString))>;

    static_assert(std::is_same_v<std::remove_const_t<ExprType>, std::string>,
                  "ExprType     " );
    static_assert(!std::is_same_v<ExprType, std::string>,
                  "   mutable ." );
}

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 , const
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(C++11)(C++11)(C++11)
 const / volatile
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(C++23)
 view constant_range
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