shared_ptr class template

Introduction
Best Practices
Synopsis
Members
Free Functions
Example
Handle/Body Idiom
Thread Safety
Frequently Asked Questions
Smart Pointer Timings
Programming Techniques

Introduction

The shared_ptr class template stores a pointer to a dynamically allocated object, typically with a C++ new-expression. The object pointed to is guaranteed to be deleted when the last shared_ptr pointing to it is destroyed or reset.

Example:

shared_ptr<X> p1( new X );
shared_ptr<void> p2( new int(5) );

shared_ptr deletes the exact pointer that has been passed at construction time, complete with its original type, regardless of the template parameter. In the second example above, when p2 is destroyed or reset, it will call delete on the original int* that has been passed to the constructor, even though p2 itself is of type shared_ptr<void> and stores a pointer of type void*.

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

Because the implementation uses reference counting, cycles of shared_ptr instances will not be reclaimed. 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. Use weak_ptr to "break cycles."

The class template is parameterized on T, the type of the object pointed to. shared_ptr and most of its member functions place no requirements on T; it is allowed to be an incomplete type, or void. Member functions that do place additional requirements (constructors, reset) are explicitly documented below.

shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*. In particular, shared_ptr<T> is implicitly convertible to shared_ptr<T const>, to shared_ptr<U> where U is an accessible base of T, and to shared_ptr<void>.

shared_ptr is now part of the C++11 Standard, as std::shared_ptr.

Starting with Boost release 1.53, shared_ptr can be used to hold a pointer to a dynamically allocated array. This is accomplished by using an array type (T[] or T[N]) as the template parameter. There is almost no difference between using an unsized array, T[], and a sized array, T[N]; the latter just enables operator[] to perform a range check on the index.

Example:

shared_ptr<double[1024]> p1( new double[1024] );
shared_ptr<double[]> p2( new double[n] );

Best Practices

A simple guideline that nearly eliminates the possibility of memory leaks is: always use a named smart pointer variable to hold the result of new. Every occurence of the new keyword in the code should have the form:

shared_ptr<T> p(new Y);

It is, of course, acceptable to use another smart pointer in place of shared_ptr above; having T and Y be the same type, or passing arguments to Y's constructor is also OK.

If you observe this guideline, it naturally follows that you will have no explicit delete statements; try/catch constructs will be rare.

Avoid using unnamed shared_ptr temporaries to save typing; to see why this is dangerous, consider this example:

void f(shared_ptr<int>, int);
int g();

void ok()
{
    shared_ptr<int> p( new int(2) );
    f( p, g() );
}

void bad()
{
    f( shared_ptr<int>( new int(2) ), g() );
}

The function ok follows the guideline to the letter, whereas bad constructs the temporary shared_ptr in place, admitting the possibility of a memory leak. Since function arguments are evaluated in unspecified order, it is possible for new int(2) to be evaluated first, g() second, and we may never get to the shared_ptrconstructor if g throws an exception. See Herb Sutter's treatment (also here) of the issue for more information.

The exception safety problem described above may also be eliminated by using the make_shared or allocate_shared factory functions defined in boost/make_shared.hpp. These factory functions also provide an efficiency benefit by consolidating allocations.

Synopsis

namespace boost {

  class bad_weak_ptr: public std::exception;

  template<class T> class weak_ptr;

  template<class T> class shared_ptr {

    public:

      typedef see below element_type;

      shared_ptr(); // never throws
      shared_ptr(std::nullptr_t); // never throws

      template<class Y> explicit shared_ptr(Y * p);
      template<class Y, class D> shared_ptr(Y * p, D d);
      template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);
      template<class D> shared_ptr(std::nullptr_t p, D d);
      template<class D, class A> shared_ptr(std::nullptr_t p, D d, A a);

      ~shared_ptr(); // never throws

      shared_ptr(shared_ptr const & r); // never throws
      template<class Y> shared_ptr(shared_ptr<Y> const & r); // never throws

      shared_ptr(shared_ptr && r); // never throws
      template<class Y> shared_ptr(shared_ptr<Y> && r); // never throws

      template<class Y> shared_ptr(shared_ptr<Y> const & r, element_type * p); // never throws

      template<class Y> shared_ptr(shared_ptr<Y> && r, element_type * p); // never throws

      template<class Y> explicit shared_ptr(weak_ptr<Y> const & r);

      template<class Y> explicit shared_ptr(std::auto_ptr<Y> & r);
      template<class Y> shared_ptr(std::auto_ptr<Y> && r);

      template<class Y, class D> shared_ptr(std::unique_ptr<Y, D> && r);

      shared_ptr & operator=(shared_ptr const & r); // never throws
      template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r); // never throws

      shared_ptr & operator=(shared_ptr const && r); // never throws
      template<class Y> shared_ptr & operator=(shared_ptr<Y> const && r); // never throws

      template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);
      template<class Y> shared_ptr & operator=(std::auto_ptr<Y> && r);

      template<class Y, class D> shared_ptr & operator=(std::unique_ptr<Y, D> && r);

      shared_ptr & operator=(std::nullptr_t); // never throws

      void reset(); // never throws

      template<class Y> void reset(Y * p);
      template<class Y, class D> void reset(Y * p, D d);
      template<class Y, class D, class A> void reset(Y * p, D d, A a);

      template<class Y> void reset(shared_ptr<Y> const & r, element_type * p); // never throws

      T & operator*() const; // never throws; only valid when T is not an array type
      T * operator->() const; // never throws; only valid when T is not an array type

      element_type & operator[](std::ptrdiff_t i) const; // never throws; only valid when T is an array type

      element_type * get() const; // never throws

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

      explicit operator bool() const; // never throws

      void swap(shared_ptr & b); // never throws
      
      template<class Y> bool owner_before(shared_ptr<Y> const & rhs) const; // never throws
      template<class Y> bool owner_before(weak_ptr<Y> const & rhs) const; // never throws
  };

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

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

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

  template<class T>
    bool operator==(shared_ptr<T> const & p, std::nullptr_t); // never throws

  template<class T>
    bool operator==(std::nullptr_t, shared_ptr<T> const & p); // never throws

  template<class T>
    bool operator!=(shared_ptr<T> const & p, std::nullptr_t); // never throws

  template<class T>
    bool operator!=(std::nullptr_t, shared_ptr<T> const & p); // never throws

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

  template<class T> typename shared_ptr<T>::element_type * get_pointer(shared_ptr<T> const & p); // never throws

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

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

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

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

  template<class E, class T, class Y>
    std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);

  template<class D, class T>
    D * get_deleter(shared_ptr<T> const & p);
}

Members

element_type

typedef ... element_type;

element_type is T when T is not an array type, and U when T is U[] or U[N].

default constructor

shared_ptr(); // never throws
shared_ptr(std::nullptr_t); // never throws

Effects: Constructs an empty shared_ptr.

Postconditions: use_count() == 0 && get() == 0.

Throws: nothing.

[The nothrow guarantee is important, since reset() is specified in terms of the default constructor; this implies that the constructor must not allocate memory.]

pointer constructor

template<class Y> explicit shared_ptr(Y * p);

Requirements: Y must be a complete type. The expression delete[] p, when T is an array type, or delete p, when T is not an array type, must be well-formed, must not invoke undefined behavior, and must not throw exceptions. When T is U[N], Y (*) [N] must be convertible to T*; when T is U[], Y (*) [] must be convertible to T*; otherwise, Y* must be convertible to T*.

Effects: When T is not an array type, constructs a shared_ptr that owns the pointer p. Otherwise, constructs a shared_ptr that owns p and a deleter of an unspecified type that calls delete[] p.

Postconditions: use_count() == 1 && get() == p. If T is not an array type and p is unambiguously convertible to enable_shared_from_this<V>* for some V, p->shared_from_this() returns a copy of *this.

Throws: std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety: If an exception is thrown, the constructor calls delete[] p, when T is an array type, or delete p, when T is not an array type.

Notes: p must be a pointer to an object that was allocated via a C++ new expression or be 0. The postcondition that use count is 1 holds even if p is 0; invoking delete on a pointer that has a value of 0 is harmless.

[This constructor is a template in order to remember the actual pointer type passed. The destructor will call delete with the same pointer, complete with its original type, even when T does not have a virtual destructor, or is void.]

constructors taking a deleter

template<class Y, class D> shared_ptr(Y * p, D d);
template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);
template<class D> shared_ptr(std::nullptr_t p, D d);
template<class D, class A> shared_ptr(std::nullptr_t p, D d, A a);

Requirements: D must be CopyConstructible. The copy constructor and destructor of D must not throw. The expression d(p) must be well-formed, must not invoke undefined behavior, and must not throw exceptions. A must be an Allocator, as described in section 20.1.5 (Allocator requirements) of the C++ Standard. When T is U[N], Y (*) [N] must be convertible to T*; when T is U[], Y (*) [] must be convertible to T*; otherwise, Y* must be convertible to T*.

Effects: Constructs a shared_ptr that owns the pointer p and the deleter d. The constructors taking an allocator a allocate memory using a copy of a.

Postconditions: use_count() == 1 && get() == p. If T is not an array type and p is unambiguously convertible to enable_shared_from_this<V>* for some V, p->shared_from_this() returns a copy of *this.

Throws: std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety: If an exception is thrown, d(p) is called.

Notes: When the the time comes to delete the object pointed to by p, the stored copy of d is invoked with the stored copy of p as an argument.

[Custom deallocators allow a factory function returning a shared_ptr to insulate the user from its memory allocation strategy. Since the deallocator is not part of the type, changing the allocation strategy does not break source or binary compatibility, and does not require a client recompilation. For example, a "no-op" deallocator is useful when returning a shared_ptr to a statically allocated object, and other variations allow a shared_ptr to be used as a wrapper for another smart pointer, easing interoperability.

The support for custom deallocators does not impose significant overhead. Other shared_ptr features still require a deallocator to be kept.

The requirement that the copy constructor of D does not throw comes from the pass by value. If the copy constructor throws, the pointer would leak.]

copy and converting constructors

shared_ptr(shared_ptr const & r); // never throws
template<class Y> shared_ptr(shared_ptr<Y> const & r); // never throws

Requires: Y* should be convertible to T*.

Effects: If r is empty, constructs an empty shared_ptr; otherwise, constructs a shared_ptr that shares ownership with r.

Postconditions: get() == r.get() && use_count() == r.use_count().

Throws: nothing.

move constructors

shared_ptr(shared_ptr && r); // never throws
template<class Y> shared_ptr(shared_ptr<Y> && r); // never throws

Requires: Y* should be convertible to T*.

Effects: Move-constructs a shared_ptr from r.

Postconditions: *this contains the old value of r. r is empty and r.get() == 0.

Throws: nothing.

aliasing constructor

template<class Y> shared_ptr(shared_ptr<Y> const & r, element_type * p); // never throws

Effects: constructs a shared_ptr that shares ownership with r and stores p.

Postconditions: get() == p && use_count() == r.use_count().

Throws: nothing.

aliasing move constructor

template<class Y> shared_ptr(shared_ptr<Y> && r, element_type * p); // never throws

Effects: Move-constructs a shared_ptr from r, while storing p instead.

Postconditions: get() == p and use_count() equals the old count of r. r is empty and r.get() == 0.

Throws: nothing.

weak_ptr constructor

template<class Y> explicit shared_ptr(weak_ptr<Y> const & r);

Requires: Y* should be convertible to T*.

Effects: Constructs a shared_ptr that shares ownership with r and stores a copy of the pointer stored in r.

Postconditions: use_count() == r.use_count().

Throws: bad_weak_ptr when r.use_count() == 0.

Exception safety: If an exception is thrown, the constructor has no effect.

auto_ptr constructors

template<class Y> shared_ptr(std::auto_ptr<Y> & r);
template<class Y> shared_ptr(std::auto_ptr<Y> && r);

Requires: Y* should be convertible to T*.

Effects: Constructs a shared_ptr, as if by storing a copy of r.release().

Postconditions: use_count() == 1.

Throws: std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety: If an exception is thrown, the constructor has no effect.

unique_ptr constructor

template<class Y, class D> shared_ptr(std::unique_ptr<Y, D> && r);

Requires: Y* should be convertible to T*.

Effects: Equivalent to shared_ptr(r.release(), r.get_deleter()) when D is not a reference type. Otherwise, equivalent to shared_ptr(r.release(), del), where del is a deleter that stores the reference rd returned from r.get_deleter() and del(p) calls rd(p).

Postconditions: use_count() == 1.

Throws: std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety: If an exception is thrown, the constructor has no effect.

destructor

~shared_ptr(); // never throws

Effects:

If *this is empty, or shares ownership with another shared_ptr instance (use_count() > 1), there are no side effects.

Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.

Otherwise, *this owns a pointer p, and delete p is called.

Throws: nothing.

assignment

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

Effects: Equivalent to shared_ptr(r).swap(*this).

Returns: *this.

Notes: The use count updates caused by the temporary object construction and destruction are not considered observable side effects, and the implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary. In particular, in the example:
shared_ptr<int> p(new int);
shared_ptr<void> q(p);
p = p;
q = p;
both assignments may be no-ops.

shared_ptr & operator=(shared_ptr && r); // never throws
template<class Y> shared_ptr & operator=(shared_ptr<Y> && r); // never throws
template<class Y> shared_ptr & operator=(std::auto_ptr<Y> && r);
template<class Y, class D> shared_ptr & operator=(std::unique_ptr<Y, D> && r);

Effects: Equivalent to shared_ptr(std::move(r)).swap(*this).

Returns: *this.

shared_ptr & operator=(std::nullptr_t); // never throws

Effects: Equivalent to shared_ptr().swap(*this).

Returns: *this.

reset

void reset(); // never throws

Effects: Equivalent to shared_ptr().swap(*this).

template<class Y> void reset(Y * p);

Effects: Equivalent to shared_ptr(p).swap(*this).

template<class Y, class D> void reset(Y * p, D d);

Effects: Equivalent to shared_ptr(p, d).swap(*this).

template<class Y, class D, class A> void reset(Y * p, D d, A a);

Effects: Equivalent to shared_ptr(p, d, a).swap(*this).

template<class Y> void reset(shared_ptr<Y> const & r, element_type * p); // never throws

Effects: Equivalent to shared_ptr(r, p).swap(*this).

indirection

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

Requirements: T should not be an array type. The stored pointer must not be 0.

Returns: a reference to the object pointed to by the stored pointer.

Throws: nothing.

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

Requirements: T should not be an array type. The stored pointer must not be 0.

Returns: the stored pointer.

Throws: nothing.

element_type & operator[](std::ptrdiff_t i) const; // never throws

Requirements: T should be an array type. The stored pointer must not be 0. i >= 0. If T is U[N], i < N.

Returns: get()[i].

Throws: nothing.

get

element_type * get() const; // never throws

Returns: the stored pointer.

Throws: nothing.

unique

bool unique() const; // never throws

Returns: use_count() == 1.

Throws: nothing.

Notes: unique() may be faster than use_count(). If you are using unique() to implement copy on write, do not rely on a specific value when the stored pointer is zero.

use_count

long use_count() const; // never throws

Returns: the number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

Throws: nothing.

Notes: use_count() is not necessarily efficient. Use only for debugging and testing purposes, not for production code.

conversions

explicit operator bool() const; // never throws

Returns: get() != 0.

Throws: nothing.

Notes: This conversion operator allows shared_ptr objects to be used in boolean contexts, like if(p && p->valid()) {}.

[The conversion to bool is not merely syntactic sugar. It allows shared_ptrs to be declared in conditions when using dynamic_pointer_cast or weak_ptr::lock.]

swap

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

Effects: Exchanges the contents of the two smart pointers.

Throws: nothing.

swap

template<class Y> bool owner_before(shared_ptr<Y> const & rhs) const; // never throws
template<class Y> bool owner_before(weak_ptr<Y> const & rhs) const; // never throws

Effects: See the description of operator<.

Throws: nothing.

Free Functions

comparison

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

Returns: a.get() == b.get().

Throws: nothing.

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

Returns: a.get() != b.get().

Throws: nothing.

template<class T>
  bool operator==(shared_ptr<T> const & p, std::nullptr_t); // never throws
template<class T>
  bool operator==(std::nullptr_t, shared_ptr<T> const & p); // never throws

Returns: p.get() == 0.

Throws: nothing.

template<class T>
  bool operator!=(shared_ptr<T> const & p, std::nullptr_t); // never throws
template<class T>
  bool operator!=(std::nullptr_t, shared_ptr<T> const & p); // never throws

Returns: p.get() != 0.

Throws: nothing.

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

Returns: an unspecified value such that

operator< is a strict weak ordering as described in section 25.3 [lib.alg.sorting] of the C++ standard;

under the equivalence relation defined by operator<, !(a < b) && !(b < a), two shared_ptr instances are equivalent if and only if they share ownership or are both empty.

Throws: nothing.

Notes: Allows shared_ptr objects to be used as keys in associative containers.

[Operator< has been preferred over a std::less specialization for consistency and legality reasons, as std::less is required to return the results of operator<, and many standard algorithms use operator< instead of std::less for comparisons when a predicate is not supplied. Composite objects, like std::pair, also implement their operator< in terms of their contained subobjects' operator<.

The rest of the comparison operators are omitted by design.]

swap

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

Effects: Equivalent to a.swap(b).

Throws: nothing.

Notes: Matches the interface of std::swap. Provided as an aid to generic programming.

[swap is defined in the same namespace as shared_ptr as this is currently the only legal way to supply a swap function that has a chance to be used by the standard library.]

get_pointer

template<class T>
  typename shared_ptr<T>::element_type * get_pointer(shared_ptr<T> const & p); // never throws

Returns: p.get().

Throws: nothing.

Notes: Provided as an aid to generic programming. Used by mem_fn.

static_pointer_cast

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

Requires: The expression static_cast<T*>( (U*)0 ) must be well-formed.

Returns: shared_ptr<T>( r, static_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

Throws: nothing.

Notes: 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.

const_pointer_cast

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

Requires: The expression const_cast<T*>( (U*)0 ) must be well-formed.

Returns: shared_ptr<T>( r, const_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

Throws: nothing.

dynamic_pointer_cast

template<class T, class U>
  shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r);

Requires: The expression dynamic_cast<T*>( (U*)0 ) must be well-formed.

Returns:

When dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) returns a nonzero value p, shared_ptr<T>(r, p);

Otherwise, shared_ptr<T>().

Throws: nothing.

reinterpret_pointer_cast

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

Requires: The expression reinterpret_cast<T*>( (U*)0 ) must be well-formed.

Returns: shared_ptr<T>( r, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

Throws: nothing.

operator<<

template<class E, class T, class Y>
    std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);

Effects: os << p.get();.

Returns: os.

get_deleter

template<class D, class T>
    D * get_deleter(shared_ptr<T> const & p);

Returns: If *this owns a deleter d of type (cv-unqualified) D, returns &d; otherwise returns 0.

Throws: nothing.

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.

Thread Safety

shared_ptr objects offer the same level of thread safety as built-in types. A shared_ptr instance can be "read" (accessed using only const operations) simultaneously by multiple threads. Different shared_ptr instances can be "written to" (accessed using mutable operations such as operator=or reset) simultaneously by multiple threads (even when these instances are copies, and share the same reference count underneath.)

Any other simultaneous accesses result in undefined behavior.

Examples:

shared_ptr<int> p(new int(42));

//--- Example 1 ---

// thread A
shared_ptr<int> p2(p); // reads p

// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe

//--- Example 2 ---

// thread A
p.reset(new int(1912)); // writes p

// thread B
p2.reset(); // OK, writes p2

//--- Example 3 ---

// thread A
p = p3; // reads p3, writes p

// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write

//--- Example 4 ---

// thread A
p3 = p2; // reads p2, writes p3

// thread B
// p2 goes out of scope: undefined, the destructor is considered a "write access"

//--- Example 5 ---

// thread A
p3.reset(new int(1));

// thread B
p3.reset(new int(2)); // undefined, multiple writes

Starting with Boost release 1.33.0, shared_ptr uses a lock-free implementation on most common platforms.

If your program is single-threaded and does not link to any libraries that might have used shared_ptr in its default configuration, you can #define the macro BOOST_SP_DISABLE_THREADS on a project-wide basis to switch to ordinary non-atomic reference count updates.

(Defining BOOST_SP_DISABLE_THREADS in some, but not all, translation units is technically a violation of the One Definition Rule and undefined behavior. Nevertheless, the implementation attempts to do its best to accommodate the request to use non-atomic updates in those translation units. No guarantees, though.)

You can define the macro BOOST_SP_USE_PTHREADS to turn off the lock-free platform-specific implementation and fall back to the generic pthread_mutex_t-based code.

Frequently Asked Questions

Q. There are several variations of shared pointers, with different tradeoffs; why does the smart pointer library supply only a single implementation? It would be useful to be able to experiment with each type so as to find the most suitable for the job at hand?

A. An important goal of shared_ptr is to provide a standard shared-ownership pointer. Having a single pointer type is important for stable library interfaces, since different shared pointers typically cannot interoperate, i.e. a reference counted pointer (used by library A) cannot share ownership with a linked pointer (used by library B.)

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. I am not convinced. Default parameters can be used where appropriate to hide the complexity. Again, why not policies?

A. Template parameters affect the type. See the answer to the first question above.

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. In addition, it is expensive to make a linked list implementation thread safe.

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.

Furthermore, the pointer returned by release() would be difficult to deallocate reliably, as the source shared_ptr could have been created with a custom deleter.

Q. Why is operator->() const, but its return value is a non-const pointer to the element type?

A. Shallow copy pointers, including raw pointers, typically don't propagate constness. It makes little sense for them to do so, as you can always obtain a non-const pointer from a const one and then proceed to modify the object through it. shared_ptr is "as close to raw pointers as possible but no closer".

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Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler. Copyright 2002-2005, 2012, 2013 Peter Dimov. Distributed under the Boost Software License, Version 1.0. See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt.