std::shared_ptr::reset(3)

💖 std::shared_ptr::reset 💖

okay bestie, this is std::shared_ptr::reset(3) from the C++ Standard Libary manual — yes, that's the manual name, and we're folding it in right here instead of doing the boring header-line copy 💅. This page is all about resetting a shared_ptr, and it's honestly iconic once you get the vibe.

✨ NAME ✨

std::shared_ptr::reset — std::shared_ptr::reset

🌸 Synopsis 🌸

          void reset() noexcept;                          (1) (since C++11)
          template< class Y >                             (2) (since C++11)
          void reset( Y* ptr );
          template< class Y, class Deleter >              (3) (since C++11)
          void reset( Y* ptr, Deleter d );
          template< class Y, class Deleter, class Alloc > (4) (since C++11)
          void reset( Y* ptr, Deleter d, Alloc alloc );

Replaces the managed object with an object pointed to by ptr. Optional deleter d can be supplied, which is later used to destroy the new object when no shared_ptr objects own it. By default, delete expression is used as deleter. Proper delete expression corresponding to the supplied type is always selected, this is the reason why the function is implemented as template using a separate parameter Y.

If *this already owns an object and it is the last shared_ptr owning it, the object is destroyed through the owned deleter.

warning bestie: if the object pointed to by ptr is already owned, the function generally results in undefined behavior. Not cute, not slay, just UB 💀

• (1) — Releases the ownership of the managed object, if any. After the call, *this manages no object. Equivalent to shared_ptr().swap(*this);.
• (2)–(4) — Replaces the managed object with an object pointed to by ptr. Y must be a complete type and implicitly convertible to T. Additionally:
  • (2) — Uses the delete expression as the deleter. A valid delete expression must be available, i.e. delete ptr must be well formed, have well-defined behavior and not throw any exceptions. Equivalent to shared_ptr<T>(ptr).swap(*this);.
  • (3) — Uses the specified deleter d as the deleter. Deleter must be callable for the type T, i.e. d(ptr) must be well formed, have well-defined behavior and not throw any exceptions. Deleter must be CopyConstructible, and its copy constructor and destructor must not throw exceptions. Equivalent to shared_ptr<T>(ptr, d).swap(*this);.
  • (4) — Same as (3), but additionally uses a copy of alloc for allocation of data for internal use. Alloc must be an Allocator. The copy constructor and destructor must not throw exceptions. Equivalent to shared_ptr<T>(ptr, d, alloc).swap(*this);.

🎀 Parameters 🎀

• ptr — pointer to an object to acquire ownership of
• d — deleter to store for deletion of the object
• alloc — allocator to use for internal allocations

💫 Return value 💫

(none) — truly no return value, just vibes 💀

😭 Exceptions 😭

• (2) — std::bad_alloc if required additional memory could not be obtained. May throw implementation-defined exception for other errors. delete ptr is called if an exception occurs.
• (3), (4) — std::bad_alloc if required additional memory could not be obtained. May throw implementation-defined exception for other errors. d(ptr) is called if an exception occurs.

🧚 Example 🧚

       // Run this code

        #include <iostream>
        #include <memory>

        struct Foo
        {
            Foo(int n = 0) noexcept : bar(n)
            {
                std::cout << "Foo::Foo(), bar = " << bar << " @ " << this <<
       '\n';
            }
            ~Foo()
            {
                std::cout << "Foo::~Foo(), bar = " << bar << " @ " << this <<
       '\n';
            }
            int getBar() const noexcept { return bar; }
        private:
            int bar;
        };

        int main()
        {
            std::cout << "1) unique ownership\n";
            {
                std::shared_ptr<Foo> sptr = std::make_shared<Foo>(100);

                std::cout << "Foo::bar = " << sptr->getBar() << ", use_count()
       = "
                          << sptr.use_count() << '\n';

                // Reset the shared_ptr without handing it a fresh instance of
       Foo.
                // The old instance will be destroyed after this call.
                std::cout << "call sptr.reset()...\n";
                sptr.reset(); // calls Foo's destructor here
                std::cout << "After reset(): use_count() = " <<
       sptr.use_count()
                          << ", sptr = " << sptr << '\n';
            }   // No call to Foo's destructor, it was done earlier in reset().

            std::cout << "\n2) unique ownership\n";
            {
                std::shared_ptr<Foo> sptr = std::make_shared<Foo>(200);

                std::cout << "Foo::bar = " << sptr->getBar() << ", use_count()
       = "
                          << sptr.use_count() << '\n';

                // Reset the shared_ptr, hand it a fresh instance of Foo.
                // The old instance will be destroyed after this call.
                std::cout << "call sptr.reset()...\n";
                sptr.reset(new Foo{222});
                std::cout << "After reset(): use_count() = " <<
       sptr.use_count()
                          << ", sptr = " << sptr << "\nLeaving the scope...\n";
            }   // Calls Foo's destructor.

            std::cout << "\n3) multiple ownership\n";
            {
                std::shared_ptr<Foo> sptr1 = std::make_shared<Foo>(300);
                std::shared_ptr<Foo> sptr2 = sptr1;
                std::shared_ptr<Foo> sptr3 = sptr2;

                std::cout << "Foo::bar = " << sptr1->getBar() << ", use_count()
       = "
                          << sptr1.use_count() << '\n';

                // Reset the shared_ptr sptr1, hand it a fresh instance of Foo.
                // The old instance will stay shared between sptr2 and sptr3.
                std::cout << "call sptr1.reset()...\n";
                sptr1.reset(new Foo{333});

                std::cout << "After reset():\n"
                          << "sptr1.use_count() = " << sptr1.use_count()
                          << ", sptr1 @ " << sptr1 << '\n'
                          << "sptr2.use_count() = " << sptr2.use_count()
                          << ", sptr2 @ " << sptr2 << '\n'
                          << "sptr3.use_count() = " << sptr3.use_count()
                          << ", sptr3 @ " << sptr3 << '\n'
                          << "Leaving the scope...\n";
            }   // Calls two destructors of: 1) Foo owned by sptr1,
                // 2) Foo shared between sptr2/sptr3.
        }

💀 Possible output 💀

        1) unique ownership
        Foo::Foo(), bar = 100 @ 0x23c5040
        Foo::bar = 100, use_count() = 1
        call sptr.reset()...
        Foo::~Foo(), bar = 100 @ 0x23c5040
        After reset(): use_count() = 0, sptr = 0

        2) unique ownership
        Foo::Foo(), bar = 200 @ 0x23c5040
        Foo::bar = 200, use_count() = 1
        call sptr.reset()...
        Foo::Foo(), bar = 222 @ 0x23c5050
        Foo::~Foo(), bar = 200 @ 0x23c5040
        After reset(): use_count() = 1, sptr = 0x23c5050
        Leaving the scope...
        Foo::~Foo(), bar = 222 @ 0x23c5050

        3) multiple ownership
        Foo::Foo(), bar = 300 @ 0x23c5080
        Foo::bar = 300, use_count() = 3
        call sptr1.reset()...
        Foo::Foo(), bar = 333 @ 0x23c5050
        After reset():
        sptr1.use_count() = 1, sptr1 @ 0x23c5050
        sptr2.use_count() = 2, sptr2 @ 0x23c5080
        sptr3.use_count() = 2, sptr3 @ 0x23c5080
        Leaving the scope...
        Foo::~Foo(), bar = 300 @ 0x23c5080
        Foo::~Foo(), bar = 333 @ 0x23c5050

🌈 See also 🌈

• constructor — constructs new shared_ptr (public member function)

and that's the whole page, bestie 🫶 — source: http://cppreference.com, version/date: 2024.06.10, page: std::shared_ptr::reset(3). Bye 💖