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std::as_const(3)	      C++ Standard Libary	      std::as_const(3)

NAME
       std::as_const - std::as_const

Synopsis
	  Defined in header <utility>
	  template< class T >					    (1)	(since
       C++17)
	  constexpr std::add_const_t<T>& as_const( T& t	) noexcept;
	  template< class T >					    (2)	(since
       C++17)
	  void as_const( const T&& ) = delete;

	  1) Forms lvalue reference to const type of t.
	  2) const rvalue reference overload is	deleted	to disallow rvalue ar-
       guments.

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

Notes
	  Feature-test macro: __cpp_lib_as_const

Example
       // Run this code

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

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

	//  mutableString.clear(); // OK
	//  constRef.clear(); // error:	'constRef' is 'const' qualified,
			      //	but 'clear' is not marked const

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

	    using	  ExprType	  =	   std::remove_reference_t<de-
       cltype(std::as_const(mutableString))>;

	    static_assert(std::is_same_v<std::remove_const_t<ExprType>,
       std::string>,
		    "ExprType should be	some kind of string." );
	    static_assert(!std::is_same_v<ExprType, std::string>,
		    "ExprType shouldn't	be a mutable string." );
	}

See also
	  is_const	  checks if a type is const-qualified
	  (C++11)	  (class template)
	  add_cv
	  add_const
	  add_volatile	  adds const or/and volatile specifiers	to  the	 given
       type
	  (C++11)	  (class template)
	  (C++11)
	  (C++11)
	  remove_cv
	  remove_const
	  remove_volatile  removes  const  or/and volatile specifiers from the
       given type
	  (C++11)	  (class template)
	  (C++11)
	  (C++11)

http://cppreference.com		  2022.07.31		      std::as_const(3)

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