std::ranges:: swap

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. 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

#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);
}

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