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 */);

Exchanges the values referenced by t and u.

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

1. (void)swap(t, u), if std::remove_cvref_t<T> or std::remove_cvref_t<U> are class or enumeration types, 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(ranges::swap(t, u)) is equal to noexcept(ranges::swap(*t, *u)).
3. Otherwise, exchanges the referenced values as if by V v(std::move(t)); t = std::move(u); u = std::move(v); if t and u are both lvalues of the same type V that satisfies std::move_constructible<V> and std::assignable_from<V&, V>, in which case noexcept(ranges::swap(t, u)) is equal to std::is_nothrow_move_constructible_v<V> && std::is_nothrow_move_assignable_v<V>.

   ranges::swap(t, u) is a constant expression if V(std::move(t)), V(std::move(u)), t = std::move(u), and u = std::move(t); are all constant expressions

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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Customization point objects

The name ranges::swap denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __swap_fn.

All instances of __swap_fn are equal. The effects of invoking different instances of type __swap_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::swap can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::swap above, __swap_fn models

• std::invocable<__swap_fn, Args...>,
• std::invocable<const __swap_fn, Args...>,
• std::invocable<__swap_fn&, Args...>, and
• std::invocable<const __swap_fn&, Args...>.
  

Otherwise, no function call operator of __swap_fn participates in overload resolution.

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)