< cpp‎ | utility
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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.


[edit] Possible implementation

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

[edit] Notes

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

[edit] Example

#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.");

[edit] See also

checks if a type is const-qualified
(class template) [edit]
adds const and/or volatile specifiers to the given type
(class template) [edit]
removes const and/or volatile specifiers from the given type
(class template) [edit]
converts a view into a constant_range
(class template) (range adaptor object)[edit]