std::as_const

 

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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; }

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_as_const	201510L	(C++17)	std::as_const

Example

Run this code

#include <cassert>
#include <string>
#include <type_traits>
#include <utility>
 
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<decltype(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 (C++11)	checks if a type is const-qualified (class template)
add_cvadd_constadd_volatile (C++11)(C++11)(C++11)	adds const and/or volatile specifiers to the given type (class template)
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)	removes const and/or volatile specifiers from the given type (class template)
ranges::as_const_viewviews::as_const (C++23)	converts a view into a constant_range (class template) (range adaptor object)

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