std::shared...owner_before(3)

🎀 NAME 🎀

okay bestie, you’re on the std::shared...owner_before(3) page from the C++ Standard Libary ✨ and the star of the show is std::shared_ptr::owner_before — literally that member function, no cap 💅

✨ Synopsis ✨

signature time, bestie — keep it exact:

template< class Y >
bool owner_before( const shared_ptr<Y>& other ) const noexcept;
template< class Y >
bool owner_before( const std::weak_ptr<Y>& other ) const noexcept;

Checks whether this shared_ptr precedes other in implementation defined owner-based (as opposed to value-based) order. The order is such that two smart pointers compare equivalent only if they are both empty or if they both own the same object, even if the values of the pointers obtained by get() are different (e.g. because they point at different subobjects within the same object). iykyk 🦋

This ordering is used to make shared and weak pointers usable as keys in associative containers, typically through std::owner_less. main character energy for containers, fr fr 👑

💅 Parameters 💅

• other — the std::shared_ptr or std::weak_ptr to be compared 🫶

🫶 Return value 🫶

true if *this precedes other, false otherwise. Common implementations compare the addresses of the control blocks. that’s the lowkey implementation tea ☕

🌸 Example 🌸

Run this code, bestie:

#include <iostream>
#include <memory>

struct Foo
{
    int n1;
    int n2;
    Foo(int a, int b) : n1(a), n2(b) {}
};

int main()
{
    auto p1 = std::make_shared<Foo>(1, 2);
    std::shared_ptr<int> p2(p1, &p1->n1);
    std::shared_ptr<int> p3(p1, &p1->n2);

    std::cout << std::boolalpha
              << "p2 < p3 " << (p2 < p3) << '\n'
              << "p3 < p2 " << (p3 < p2) << '\n'
              << "p2.owner_before(p3) " << p2.owner_before(p3) << '\n'
              << "p3.owner_before(p2) " << p3.owner_before(p2) << '\n';

    std::weak_ptr<int> w2(p2);
    std::weak_ptr<int> w3(p3);
    std::cout
//            << "w2 < w3 " << (w2 < w3) << '\n' // won't compile
//            << "w3 < w2 " << (w3 < w2) << '\n' // won't compile
              << "w2.owner_before(w3) " << w2.owner_before(w3) << '\n'
              << "w3.owner_before(w2) " << w3.owner_before(w2) << '\n';
}

🍓 Output 🍓

p2 < p3 true
p3 < p2 false
p2.owner_before(p3) false
p3.owner_before(p2) false
w2.owner_before(w3) false
w3.owner_before(w2) false

💀 Defect reports 💀

bestie, defect report tea: the following behavior-changing defect reports were applied retroactively to previously published C++ standards ☕✨

• DR: LWG 2873
• Applied to: C++11
• Behavior as published: owner_before was not declared noexcept
• Correct behavior: declared noexcept

👑 See also 👑

• owner_less — provides mixed-type owner-based ordering of shared and weak pointers (C++11) (class template) 🌈

source/version/date vibes, wrapped up cute: http://cppreference.com • 2024.06.10 • std::shared...owner_before(3) 🫶💖