std::as_const

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

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Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

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initializer_list (C++11)

bitset

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Optional and any

optional (C++17)

any (C++17)

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pair

tuple (C++11)

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piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Type operations

declval (C++11)

as_const (C++17)

 

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

Run this code

#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

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 or/and volatile specifiers to the given type (class template)
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)	removes const or/and volatile specifiers from the given type (class template)