std::ranges::swap

 

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Defined in header <concepts>
namespace ranges { inline namespace /* unspecified */ { inline constexpr /* unspecified */ swap = /* unspecified */; } }		(since C++20) (customization point object)
Call signature
template< class T, class U > constexpr void ranges::swap( T&& t, U&& u ) noexcept(/* see below */);		(since C++20)

See More

Exchanges the values referenced by t and u.

ranges::swap(t, u) is expression-equivalent to:

1. (void)swap(t, u), if t or u has class or enumeration type, and that expression is valid, where the overload resolution is performed within namespace std::ranges with the additional candidate template<class T> void swap(T&, T&) = delete;.
   • If the function selected by overload resolution does not exchange the values referenced by t and u, the program is ill-formed; no diagnostic required.
2. Otherwise, (void)ranges::swap_ranges(t, u), if t and u are lvalue arrays of equal extent (but possibly different element types) and ranges::swap(*t, *u) is a valid expression, except that noexcept((void)ranges::swap_ranges(t, u)) is equal to noexcept(ranges::swap(*t, *u)).
3. Otherwise, an expression which exchanges the referenced values of t and u, if they are both lvalues of the same type V that models std::move_constructible<V> and std::assignable_from<V&, V>.
   • The result of applying the noexcept operator to that expression is equal to std::is_nothrow_move_constructible_v<V> && std::is_nothrow_move_assignable_v<V>.
   • That expression is a constant expression if
     • V is a LiteralType,
     • both t = std::move(u)) and u = std::move(t) are constant subexpressions, and
     • the full-expressions of the initializers in the following declarations are constant subexpressions:
       • V v1(std::move(t));
       • V v2(std::move(u));
4. Otherwise, ranges::swap(t, u) is ill-formed, which can result in substitution failure when ranges::swap(t, u) appears in the immediate context of a template instantiation.

Customization point objects

The name ranges::swap denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Example

Run this code

#include <array>
#include <concepts>
#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>

void print(std::string_view name, 
           std::ranges::common_range auto const& p, 
           std::ranges::common_range auto const& q)
{
    std::cout << name << "1{ ";
    for (auto const& i : p)
        std::cout << i << ' ';
    std::cout << "}, " << name << "2{ ";
    for (auto const& i : q)
        std::cout << i << ' ';
    std::cout << "}\n";
}

void print(std::string_view name, int p, int q)
{
    std::cout << name << "1 = " << p << ", " << name << "2 = " << q << '\n';
}

struct IntLike
{
    int v;
};

void swap(IntLike& lhs, int& rhs)
{
    std::swap(lhs.v, rhs);
}

void swap(int& lhs, IntLike& rhs)
{
    std::swap(lhs, rhs.v);
}

std::ostream& operator<<(std::ostream& out, IntLike i)
{
    return out << i.v;
}

int main()
{
    std::vector a1{10, 11, 12}, a2{13, 14};
    std::ranges::swap(a1, a2);
    print("a", a1, a2);
    
    std::array b1{15, 16, 17}, b2{18, 19, 20};
    std::ranges::swap(b1, b2);
    print("b", b1, b2);
    
    // std::array c1{1, 2, 3}; std::array c2{4, 5};
    // std::ranges::swap(c1, c2); // error: no swap found by ADL
    
    int d1[]{21, 22, 23}, d2[]{24, 25, 26};
    std::ranges::swap(d1, d2);
    print("d", d1, d2);
    
    // int e1[]{1, 2, 3}, e2[]{4, 5};
    // std::ranges::swap(e1, e2); // error: extents mismatch
    
    // char f1[]{1, 2, 3};
    // int  f2[]{4, 5, 6};
    // std::ranges::swap(f1, f2); // error: no swap(*f1, *f2) found by ADL
    
    IntLike g1[]{1, 2, 3};
    int     g2[]{4, 5, 6};
    std::ranges::swap(g1, g2); // heterogeneous swap supported
    print("g", g1, g2);
    
    int h1{27}, h2{28};
    std::ranges::swap(h1, h2);
    print("h", h1, h2);
}

Output:

a1{ 13 14 }, a2{ 10 11 12 }
b1{ 18 19 20 }, b2{ 15 16 17 }
d1{ 24 25 26 }, d2{ 21 22 23 }
g1{ 4 5 6 }, g2{ 1 2 3 }
h1 = 28, h2 = 27

See also

swappableswappable_with (C++20)	specifies that a type can be swapped or that two types can be swapped with each other (concept)
swap	swaps the values of two objects (function template)