# shared_ptr class template

Introduction
Synopsis
Members
Free Functions
Example
Handle/Body Idiom
Frequently Asked Questions
Smart Pointer Timings

## Introduction

The shared_ptr class template stores a pointer to a dynamically allocated object. (Dynamically allocated objects are allocated with the C++ new expression.) The object pointed to is guaranteed to be deleted when the last shared_ptr pointing to it is destroyed or reset. See the example.

Every shared_ptr meets the CopyConstructible and Assignable requirements of the C++ Standard Library, and so can be used in standard library containers. Comparison operators are supplied so that shared_ptr works with the standard library's associative containers.

Normally, a shared_ptr cannot correctly hold a pointer to a dynamically allocated array. See shared_array for that usage.

Because the implementation uses reference counting, shared_ptr will not work correctly with cyclic data structures. For example, if main() holds a shared_ptr to A, which directly or indirectly holds a shared_ptr back to A, A's use count will be 2. Destruction of the original shared_ptr will leave A dangling with a use count of 1.

The class template is parameterized on T, the type of the object pointed to. T must meet the smart pointer common requirements. T may be void, but in that case, either an explicit delete function must be passed in, or the pointed-to object must have a trivial destructor.

## Synopsis

    namespace boost {

      template<typename T> class shared_ptr {

        public:
          typedef T element_type;

          explicit shared_ptr(T * p = 0);
          template<typename D> shared_ptr(T * p, D d);
          ~shared_ptr(); // never throws

          shared_ptr(shared_ptr const & r); // never throws
          template<typename Y> shared_ptr(shared_ptr<Y> const & r); // never throws
          template<typename Y> shared_ptr(std::auto_ptr<Y> & r);

          shared_ptr & operator=(shared_ptr const & r); // never throws
          template<typename Y> shared_ptr & operator=(shared_ptr<Y> const & r); // never throws
          template<typename Y> shared_ptr & operator=(std::auto_ptr<Y> & r);

          void reset(T * p = 0); // never throws
          template<typename D> void reset(T * p, D d); // never throws

          T & operator*() const; // never throws
          T * operator->() const; // never throws
          T * get() const; // never throws

          bool unique() const; // never throws
          long use_count() const; // never throws

          void swap(shared_ptr<T> & b); // never throws
      };

      template<typename T, typename U>
        bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
      template<typename T, typename U>
        bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
      template<typename T, typename U>
        bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws

      template<typename T> void swap(shared_ptr<T> & a, shared_ptr<T> & b); // never throws

      template<typename T, typename U>
        shared_ptr<T> shared_static_cast(shared_ptr<U> const & r); // never throws
      template<typename T, typename U>
        shared_ptr<T> shared_dynamic_cast(shared_ptr<U> const & r);

    }

## Members

### element_type

    typedef T element_type;

Provides the type of the stored pointer.

### constructors

    explicit shared_ptr(T * p = 0);

Constructs a shared_ptr, storing a copy of p, which must be a pointer to an object that was allocated via a C++ new expression or be 0. Afterwards, the use count is 1 (even if p == 0; see ~shared_ptr). The only exception which may be thrown by this constructor is std::bad_alloc. If an exception is thrown, delete p is called.

    template<typename D> shared_ptr(T * p, D d);

Constructs a shared_ptr, storing a copy of p and of d. Afterwards, the use count is 1. D's copy constructor and destructor must not throw. When the the time comes to delete the object pointed to by p, the object d is used in the statement d(p). Invoking the object d with parameter p in this way must not throw. The only exception which may be thrown by this constructor is std::bad_alloc. If an exception is thrown, d(p) is called.

    shared_ptr(shared_ptr const & r); // never throws
    template<typename Y> shared_ptr(shared_ptr<Y> const & r); // never throws
    template<typename Y> shared_ptr(std::auto_ptr<Y> & r);

Constructs a shared_ptr, as if by storing a copy of the pointer stored in r. Afterwards, the use count for all copies is 1 more than the initial use count, or 1 in the auto_ptr case. In the auto_ptr case, r.release() is called. The only exception which may be thrown is std::bad_alloc, which may be thrown during construction from auto_ptr. If an exception is thrown, the constructor has no effect.

### destructor

    ~shared_ptr(); // never throws

Decrements the use count. Then, if the use count is 0, deletes the object pointed to by the stored pointer. Note that delete on a pointer with a value of 0 is harmless. T need not be a complete type. The guarantee that this does not throw exceptions depends on the requirement that the deleted object's destructor does not throw exceptions. See the smart pointer common requirements.

### assignment

    shared_ptr & operator=(shared_ptr const & r); // never throws
    template<typename Y> shared_ptr & operator=(shared_ptr<Y> const & r); // never throws
    template<typename Y> shared_ptr & operator=(std::auto_ptr<Y> & r);

Constructs a new shared_ptr as described above, then replaces this shared_ptr with the new one, destroying the replaced object. The only exception which may be thrown is std::bad_alloc, which may be thrown during assignment from auto_ptr. If an exception is thrown, the assignment has no effect.

### reset

    void reset(T * p = 0);

Constructs a new shared_ptr as described above, then replaces this shared_ptr with the new one, destroying the replaced object. The only exception which may be thrown is std::bad_alloc. If an exception is thrown, delete p is called.

    template<typename D> void reset(T * p, D d);

Constructs a new shared_ptr as described above, then replaces this shared_ptr with the new one, destroying the replaced object. D's copy constructor must not throw. The only exception which may be thrown is std::bad_alloc. If an exception is thrown, d(p) is called.

### indirection

    T & operator*() const; // never throws

Returns a reference to the object pointed to by the stored pointer. Behavior is undefined if the stored pointer is 0.

    T * operator->() const; // never throws

Returns the stored pointer. Behavior is undefined if the stored pointer is 0.

### get

    T * get() const; // never throws

Returns the stored pointer. T need not be a complete type. See the smart pointer common requirements.

### unique

    bool unique() const; // never throws

Returns true if no other shared_ptr is sharing ownership of the stored pointer, false otherwise. T need not be a complete type. See the smart pointer common requirements.

### use_count

    long use_count() const; // never throws

Returns the number of shared_ptr objects sharing ownership of the stored pointer. T need not be a complete type. See the smart pointer common requirements.

Because use_count is not necessarily efficient to implement for implementations of shared_ptr that do not use an explicit reference count, it might be removed from some future version. Thus it should be used for debugging purposes only, and not production code.

### swap

    void swap(shared_ptr & b); // never throws

Exchanges the contents of the two smart pointers. T need not be a complete type. See the smart pointer common requirements.

## Free Functions

### comparison

    template<typename T, typename U>
      bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
    template<typename T, typename U>
      bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
    template<typename T, typename U>
      bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws

Compares the stored pointers of the two smart pointers. T need not be a complete type. See the smart pointer common requirements.

The operator< overload is provided to define an ordering so that shared_ptr objects can be used in associative containers such as std::map. The implementation uses std::less<T *> to perform the comparison. This ensures that the comparison is handled correctly, since the standard mandates that relational operations on pointers are unspecified (5.9 [expr.rel] paragraph 2) but std::less<> on pointers is well-defined (20.3.3 [lib.comparisons] paragraph 8).

### swap

    template<typename T>
      void swap(shared_ptr<T> & a, shared_ptr<T> & b) // never throws

Equivalent to a.swap(b). Matches the interface of std::swap. Provided as an aid to generic programming.

### shared_static_cast

    template<typename T, typename U>
      shared_ptr<T> shared_static_cast(shared_ptr<U> const & r); // never throws

Perform a static_cast on the stored pointer, returning another shared_ptr. The resulting smart pointer will share its use count with the original pointer.

Note that the seemingly equivalent expression

> `shared_ptr<T>(static_cast<T*>(r.get()))`

will eventually result in undefined behavior, attempting to delete the same object twice.

### shared_dynamic_cast

    template<typename T, typename U>
      shared_ptr<T> shared_dynamic_cast(shared_ptr<U> const & r);

Perform a dynamic_cast on the stored pointer, returning another shared_ptr. The resulting smart pointer will share its use count with the original pointer unless the result of the cast is 0. The only exception which may be thrown is std::bad_alloc, which may be thrown during the construction of the new shared_ptr if the result of the cast is 0. If an exception is thrown, the cast has no effect.

Note that the seemingly equivalent expression

> `shared_ptr<T>(dynamic_cast<T*>(r.get()))`

will eventually result in undefined behavior, attempting to delete the same object twice.

## Example

See shared_ptr_example.cpp for a complete example program. The program builds a std::vector and std::set of shared_ptr objects.

Note that after the containers have been populated, some of the shared_ptr objects will have a use count of 1 rather than a use count of 2, since the set is a std::set rather than a std::multiset, and thus does not contain duplicate entries. Furthermore, the use count may be even higher at various times while push_back and insert container operations are performed. More complicated yet, the container operations may throw exceptions under a variety of circumstances. Getting the memory management and exception handling in this example right without a smart pointer would be a nightmare.

## Handle/Body Idiom

One common usage of shared_ptr is to implement a handle/body (also called pimpl) idiom which avoids exposing the body (implementation) in the header file.

The shared_ptr_example2_test.cpp sample program includes a header file, shared_ptr_example2.hpp, which uses a shared_ptr<> to an incomplete type to hide the implementation. The instantiation of member functions which require a complete type occurs in the shared_ptr_example2.cpp implementation file. Note that there is no need for an explicit destructor. Unlike ~scoped_ptr, ~shared_ptr does not require that T be a complete type.

## Frequently Asked Questions

Q. Why doesn't shared_ptr have template parameters supplying traits or policies to allow extensive user customization?
A. Parameterization discourages users. The shared_ptr template is carefully crafted to meet common needs without extensive parameterization. Some day a highly configurable smart pointer may be invented that is also very easy to use and very hard to misuse. Until then, shared_ptr is the smart pointer of choice for a wide range of applications. (Those interested in policy based smart pointers should read Modern C++ Design by Andrei Alexandrescu.)

Q. Why doesn't shared_ptr use a linked list implementation?
A. A linked list implementation does not offer enough advantages to offset the added cost of an extra pointer. See timings page.

Q. Why doesn't shared_ptr (or any of the other Boost smart pointers) supply an automatic conversion to T*?
A. Automatic conversion is believed to be too error prone.

Q. Why does shared_ptr supply use_count()?
A. As an aid to writing test cases and debugging displays. One of the progenitors had use_count(), and it was useful in tracking down bugs in a complex project that turned out to have cyclic-dependencies.

Q. Why doesn't shared_ptr specify complexity requirements?
A. Because complexity requirements limit implementors and complicate the specification without apparent benefit to shared_ptr users. For example, error-checking implementations might become non-conforming if they had to meet stringent complexity requirements.

Q. Why doesn't shared_ptr provide a release() function?
A. shared_ptr cannot give away ownership unless it's unique() because the other copy will still destroy the object.

Consider:

>     shared_ptr<int> a(new int);
>     shared_ptr<int> b(a); // a.use_count() == b.use_count() == 2

>     int * p = a.release();

>     // Who owns p now? b will still call delete on it in its destructor.

Q. Why doesn't shared_ptr provide (your pet feature here)?
A. Because (your pet feature here) would mandate a reference counted implementation or a linked list implementation, or some other specific implementation. This is not the intent.

Revised 1 February 2002

Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.