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std::as_const

Defined in header <utility> 
template <class T>
constexpr std::add_const_t<T>& as_const(T& t) noexcept;
 (1) (since C++17) 
template <class T>
void as_const(const T&&) = delete;
 (2) (since C++17)
1) Forms lvalue reference to const type of t
2) const rvalue reference overload is deleted to disallow rvalue arguments

Possible implementation

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

Example

#include <string>
#include <cassert>
#include <utility>
#include <type_traits>
 
int main()
{
    std::string mutableString = "Hello World!";
    const std::string& constView = std::as_const(mutableString);
 
    assert( &constView == &mutableString );
    assert( &std::as_const( mutableString ) == &mutableString );
 
    using WhatTypeIsIt = std::remove_reference_t<decltype(std::as_const(mutableString))>;
 
    static_assert(std::is_same<std::remove_const_t<WhatTypeIsIt>, std::string>::value,
            "WhatTypeIsIt should be some kind of string." );
    static_assert(!std::is_same< WhatTypeIsIt, std::string >::value,
            "WhatTypeIsIt shouldn't be a mutable string." );
}

See also

(C++11)
 checks if a type is const-qualified
(class template)
(C++11)(C++11)(C++11)
 adds const or/and volatile specifiers to the given type
(class template)
(C++11)(C++11)(C++11)
 removes const or/and volatile specifiers from the given type
(class template)

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