NAME¶

std::ranges::swap - std::ranges::swap

Synopsis¶

Defined in header <concepts>
namespace ranges {

inline namespace /* unspecified */ {
inline constexpr /* unspecified */ swap = /* (since C++20)
unspecified */; (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¶

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