std::forward_like

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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 is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Defined in header <utility>
template< class T, class U > constexpr auto&& forward_like( U&& x ) noexcept;		(since C++23)

Returns a reference to x which has similar properties to T&&.

The return type is determined as below:

1. If std::remove_reference_t<T> is a const-qualified type, then the referenced type of the return type is const std::remove_reference_t<U>. Otherwise, the referenced type is std::remove_reference_t<U>.
2. If T&& is an lvalue reference type, then the return type is also an lvalue reference type. Otherwise, the return type is an rvalue reference type.

If T is not a referenceable type, the program is ill-formed.

Parameters

x

-

a value needs to be forwarded like type T

Return value

A reference to x of the type determined as above.

Notes

Like std::forward, std::move, and std::as_const, std::forward_like is a type cast that only influences the value category of an expression, or potentially adds const-qualification.

When m is an actual member and thus o.m a valid expression, this is usually spelled as std::forward<decltype(o)>(o).m in C++20 code.

This leads to three possible models, called merge, tuple, and language.

• merge: merge the const qualifiers, and adopt the value category of the Owner.
• tuple: what std::get<0>(Owner) does, assuming Owner is a std::tuple<Member>.
• language: what std::forward<decltype(Owner)>(o).m does.

The main scenario that std::forward_like caters to is adapting “far” objects. Neither the tuple nor the language scenarios do the right thing for that main use-case, so the merge model is used for std::forward_like.

Feature-test macro	Value	Std	Feature
__cpp_lib_forward_like	202207L	(C++23)	std::forward_like

Possible implementation

template<class T, class U> constexpr auto&& forward_like(U&& x) noexcept { constexpr bool is_adding_const = std::is_const_v<std::remove_reference_t<T>>; if constexpr (std::is_lvalue_reference_v<T&&>) { if constexpr (is_adding_const) return std::as_const(x); else return static_cast<U&>(x); } else { if constexpr (is_adding_const) return std::move(std::as_const(x)); else return std::move(x); } }

Example

Run this code

#include <cstddef>
#include <iostream>
#include <memory>
#include <optional>
#include <type_traits>
#include <utility>
#include <vector>
 
struct TypeTeller
{
    void operator()(this auto&& self)
    {
        using SelfType = decltype(self);
        using UnrefSelfType = std::remove_reference_t<SelfType>;
        if constexpr (std::is_lvalue_reference_v<SelfType>)
        {
            if constexpr (std::is_const_v<UnrefSelfType>)
                std::cout << "const lvalue\n";
            else
                std::cout << "mutable lvalue\n";
        }
        else
        {
            if constexpr (std::is_const_v<UnrefSelfType>)
                std::cout << "const rvalue\n";
            else
                std::cout << "mutable rvalue\n";
        }
    }
};
 
struct FarStates
{
    std::unique_ptr<TypeTeller> ptr;
    std::optional<TypeTeller> opt;
    std::vector<TypeTeller> container;
 
    auto&& from_opt(this auto&& self)
    {
        return std::forward_like<decltype(self)>(self.opt.value());
        // It is OK to use std::forward<decltype(self)>(self).opt.value(),
        // because std::optional provides suitable accessors.
    }
 
    auto&& operator[](this auto&& self, std::size_t i)
    {
        return std::forward_like<decltype(self)>(self.container.at(i));
        // It is not so good to use std::forward<decltype(self)>(self)[i], because
        // containers do not provide rvalue subscript access, although they could.
    }
 
    auto&& from_ptr(this auto&& self)
    {
        if (!self.ptr)
            throw std::bad_optional_access{};
        return std::forward_like<decltype(self)>(*self.ptr);
        // It is not good to use *std::forward<decltype(self)>(self).ptr, because
        // std::unique_ptr<TypeTeller> always dereferences to a non-const lvalue.
    }
};
 
int main()
{
    FarStates my_state
    {
        .ptr{std::make_unique<TypeTeller>()},
        .opt{std::in_place, TypeTeller{}},
        .container{std::vector<TypeTeller>(1)},
    };
 
    my_state.from_ptr()();
    my_state.from_opt()();
    my_state[0]();
 
    std::cout << '\n';
 
    std::as_const(my_state).from_ptr()();
    std::as_const(my_state).from_opt()();
    std::as_const(my_state)[0]();
 
    std::cout << '\n';
 
    std::move(my_state).from_ptr()();
    std::move(my_state).from_opt()();
    std::move(my_state)[0]();
 
    std::cout << '\n';
 
    std::move(std::as_const(my_state)).from_ptr()();
    std::move(std::as_const(my_state)).from_opt()();
    std::move(std::as_const(my_state))[0]();
 
    std::cout << '\n';
}

Output:

mutable lvalue
mutable lvalue
mutable lvalue
 
const lvalue
const lvalue
const lvalue
 
mutable rvalue
mutable rvalue
mutable rvalue
 
const rvalue
const rvalue
const rvalue

See also

move (C++11)	converts the argument to an xvalue (function template)
forward (C++11)	forwards a function argument and use the type template argument to preserve its value category (function template)
as_const (C++17)	obtains a reference to const to its argument (function template)