# Class scoped_ptr

Class scoped_ptr 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, either on destruction of the scoped_ptr, or via an explicit scoped_ptr::reset(). See example.

Class scoped_ptr is a simple solution for simple needs. It supplies a basic "resource acquisition is initialization" facility, without shared-ownership or transfer-of-ownership semantics. Both its name and enforcement of semantics (by being noncopyable) signal its intent to retain ownership solely within the current scope. Because it is noncopyable, it is safer than shared_ptr or std::auto_ptr for pointers which should not be copied.

Because scoped_ptr is so simple, in its usual implementation every operation is as fast as for a built-in pointer and it has no more space overhead that a built-in pointer. (Because of the "complete type" requirement for delete and reset members, they may have one additional function call overhead in certain idioms. See Handle/Body Idiom.)

Class scoped_ptr cannot be used in C++ Standard Library containers. See shared_ptr or std::auto_ptr if scoped_ptr does not meet your needs.

Class scoped_ptr cannot correctly hold a pointer to a dynamically allocated array. See scoped_array for that usage.

The class is a template parameterized on `T`, the type of the object pointed to. `T` must meet the smart pointer common requirements.

## Class scoped_ptr Synopsis

    #include <boost/smart_ptr.hpp>
    namespace boost {

    template<typename T> class scoped_ptr : noncopyable {

     public:
       typedef T element_type;

       explicit scoped_ptr( T* p=0 );  // never throws
       ~scoped_ptr();

       void reset( T* p=0 );

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

## Class scoped_ptr Members

### scoped_ptr element_type

    typedef T element_type;

Provides the type of the stored pointer.

### scoped_ptr constructors

    explicit scoped_ptr( T* p=0 );  // never throws

T is not required be a complete type. See Common Requirements.

Constructs a `scoped_ptr`, storing a copy of `p`, which must have been allocated via a C++ `new` expression or be 0.

### scoped_ptr destructor

    ~scoped_ptr();

Deletes the object pointed to by the stored pointer. Note that in C++, `delete` on a pointer with a value of 0 is harmless.

Does not throw exceptions.

### scoped_ptr reset

    void reset( T* p=0 );

If p is not equal to the stored pointer, deletes the object pointed to by the stored pointer and then stores a copy of p, which must have been allocated via a C++ `new` expression or be 0.

Does not throw exceptions.

### scoped_ptr operator*

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

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

### scoped_ptr operator-> and get

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

T is not required by get() be a complete type. See Common Requirements.

Both return the stored pointer.

## Class scoped_ptr examples

    #include <iostream>
    #include <boost/smart_ptr.h>

    struct Shoe { ~Shoe(){ std::cout << "Buckle my shoe" << std::endl; } };

    class MyClass {
        boost::scoped_ptr<int> ptr;
      public:
        MyClass() : ptr(new int) { *ptr = 0; }
        int add_one() { return ++*ptr; }
        };

    void main() {
        boost::scoped_ptr<Shoe> x(new Shoe);
        MyClass my_instance;
        std::cout << my_instance.add_one() << std::endl;
        std::cout << my_instance.add_one() << std::endl;
        }

The example program produces the beginning of a child's nursery rhyme as output:

>     1
>     2
>     Buckle my shoe

## Rationale

The primary reason to use scoped_ptr rather than auto_ptr is to let readers of your code know that you intend "resource acquisition is initialization" to be applied only for the current scope, and have no intent to transfer ownership.

A secondary reason to use scoped_ptr is to prevent a later maintenance programmer from adding a function that actually transfers ownership by returning the auto_ptr (because the maintenance programmer saw auto_ptr, and assumed ownership could safely be transferred.)

Think of bool vs int. We all know that under the covers bool is usually just an int. Indeed, some argued against including bool in the C++ standard because of that. But by coding bool rather than int, you tell your readers what your intent is. Same with scoped_ptr - you are signaling intent.

It has been suggested that boost::scoped_ptr<T> is equivalent to std::auto_ptr<T> const. Ed Brey pointed out, however, that reset() will not work on a std::auto_ptr<T> const.

## Handle/Body Idiom

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

The scoped_ptr_example_test.cpp sample program includes a header file, scoped_ptr_example.hpp, which uses a scoped_ptr<> to an incomplete type to hide the implementation. The instantiation of member functions which require a complete type occurs in the scoped_ptr_example.cpp implementation file.

## FAQ

Q. Why doesn't scoped_ptr have a release() member?
A. Because the whole point of scoped_ptr is to signal intent not to transfer ownership. Use std::auto_ptr if ownership transfer is required.

Revised 24 May 2001

© Copyright Greg Colvin and Beman Dawes 1999. 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.