std::ranges::swap(3)

💅 std::ranges::swap — bestie edition 💅

okay so this is the manual page for std::ranges::swap from the C++ Standard Libary (yes, that spelling is literally in the header, no cap 💀). It’s giving C++20 customization-point-object main character energy, and it lives in the <concepts> header. Let’s get into it ✨💖

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🌸 NAME 🌸

std::ranges::swap — literally std::ranges::swap (the name ate and left no crumbs 💅).

✨ Synopsis ✨

okay bestie, here’s the setup. Defined in header <concepts>, and the namespace situation is this whole little nest 🪺:

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 )             (since C++20)
noexcept(/* see below */);

It exchanges the values referenced by t and u, periodt 💫.

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. (yes bestie, “no diagnostic required” is exactly as unhinged as it sounds 😭💀)
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. slay, copyable icon behavior 💅✨

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 — (C++20) (concept) — specifies that a type can be swapped or that two types can be swapped with each other. The page also has the name swappable_with right before the with each other bit, so I’m keeping both visible bestie 👀💫
• swappable_with — (C++20) (concept) — the exact little fragment next to this name is with each other.
• swap — (function template) — swaps the values of two objects.

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okay that’s the whole page, bestie! source: http://cppreference.com, version/date: 2024.06.10, and this was the manual page for std::ranges::swap(3) 🫶💖✨