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< title > scoped_ptr< / title >
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< h1 > < img src = "../../c++boost.gif" alt = "c++boost.gif (8819 bytes)" align = "center" width = "277" height = "86" > Class
< a name = "scoped_ptr" > scoped_ptr< / a > < / h1 >
< p > Class < strong > scoped_ptr< / strong > stores a pointer to a dynamically allocated
object. (Dynamically allocated objects are allocated with the C++ < tt > new< / tt >
expression.) The object pointed to is guaranteed to be deleted,
either on destruction of the < strong > scoped_ptr< / strong > , or via an explicit < strong > scoped_ptr::reset()< / strong > .
See < a href = "#scoped_ptr_example" > example< / a > .< / p >
< p > Class< strong > scoped_ptr< / strong > is a simple solution for simple
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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 < a href = "../utility/utility.htm#class noncopyable" > noncopyable< / a > )
signal its intent to retain ownership solely within the current scope. By
being < a href = "../utility/utility.htm#class noncopyable" > noncopyable< / a > , it is
safer than < b > shared_ptr< / b > or std::auto_ptr for pointers which should not be
copied.< / p >
< p > Because < strong > scoped_ptr< / strong > 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.< / p >
< p > Class< strong > scoped_ptr< / strong > cannot be used in C++ Standard Library containers. See < a href = "shared_ptr.htm" > < strong > shared_ptr< / strong > < / a >
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or std::auto_ptr if < strong > scoped_ptr< / strong > does not meet your needs.< / p >
< p > Class< strong > scoped_ptr< / strong > cannot correctly hold a pointer to a
dynamically allocated array. See < a href = "scoped_array.htm" > < strong > scoped_array< / strong > < / a >
for that usage.< / p >
< p > The class is a template parameterized on < tt > T< / tt > , the type of the object
pointed to. < tt > T< / tt > must meet the smart pointer < a href = "smart_ptr.htm#Common requirements" > common
requirements< / a > .< / p >
< h2 > Class scoped_ptr Synopsis< / h2 >
< pre > #include < boost/smart_ptr.hpp>
namespace boost {
template< typename T> class scoped_ptr : < a href = "../utility/utility.htm#class noncopyable" > noncopyable< / a > {
public:
typedef T < a href = "#scoped_ptr_element_type" > element_type< / a > ;
explicit < a href = "#scoped_ptr_ctor" > scoped_ptr< / a > ( T* p=0 ); // never throws
< strong > < / strong > < a href = "#scoped_ptr_~scoped_ptr" > ~scoped_ptr< / a > ();
void < a href = "#scoped_ptr_reset" > reset< / a > ( T* p=0 );
T& < a href = "#scoped_ptr_operator*" > operator*< / a > () const; // never throws
T* < a href = "#scoped_ptr_operator->" > operator-> < / a > () const; // never throws
T* < a href = "#scoped_ptr_get" > get< / a > () const; // never throws
};
}< / pre >
< h2 > Class scoped_ptr Members< / h2 >
< h3 > scoped_ptr < a name = "scoped_ptr_element_type" > element_type< / a > < / h3 >
< pre > typedef T element_type;< / pre >
< p > Provides the type of the stored pointer.< / p >
< h3 > < a name = "scoped_ptr_ctor" > scoped_ptr constructors< / a > < / h3 >
< pre > explicit scoped_ptr( T* p=0 ); // never throws< / pre >
< p > Constructs a < tt > scoped_ptr< / tt > , storing a copy of < tt > p< / tt > , which must
have been allocated via a C++ < tt > new< / tt > expression or be 0..< / p >
< h3 > < a name = "scoped_ptr_~scoped_ptr" > scoped_ptr destructor< / a > < / h3 >
< pre > ~scoped_ptr();< / pre >
< p > Deletes the object pointed to by the stored pointer. Note that in C++, < tt > delete< / tt >
on a pointer with a value of 0 is harmless.< / p >
< p > Does not throw exceptions.< / p >
< h3 > scoped_ptr < a name = "scoped_ptr_reset" > reset< / a > < / h3 >
< pre > void reset( T* p=0 );< / pre >
< p > 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++ < tt > new< / tt > expression or be 0.< / p >
< p > Does not throw exceptions.< / p >
< h3 > scoped_ptr < a name = "scoped_ptr_operator*" > operator*< / a > < / h3 >
< pre > T& operator*() const; // never throws< / pre >
< p > Returns a reference to the object pointed to by the stored pointer.< / p >
< h3 > scoped_ptr < a name = "scoped_ptr_operator->" > operator-> < / a > and < a name = "scoped_ptr_get" > get< / a > < / h3 >
< pre > T* operator-> () const; // never throws
T* get() const; // never throws< / pre >
< p > Both return the stored pointer.< / p >
< h2 > Class < a name = "scoped_ptr_example" > scoped_ptr example< / a > s< / h2 >
< pre > #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;
}< / pre >
< p > The example program produces the beginning of a child's nursery rhyme as
output:< / p >
< blockquote >
< pre > 1
2
Buckle my shoe< / pre >
< / blockquote >
< h2 > Handle/Body Idiom< / h2 >
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< p > One common usage of < b > scoped_ptr< / b > is to implement a handle/body
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structure which avoids exposing the body (implementation) in the header file:< / p >
< pre > class handle
{
public: // simple forwarding functions to the body class
void f();
void g(int);
private:
friend class body; //incomplete class hides implementation
boost::scoped_ptr< body> imp;
};< / pre >
< p > This code requires that < code > class body< / code > have a trivial destructor to
avoid undefined behavior. This is because the definition of < code > class
body< / code > is not visible at the time scoped_ptr< > deletes it. See ISO
5.3.5/5. Note that some compilers will issue a warning even though the
above code is well defined.< / p >
< hr >
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< p > Revised <!-- webbot bot="Timestamp" s - type="EDITED" s - format="%d %B %Y" startspan --> 21 May 2001<!-- webbot bot="Timestamp" endspan i - checksum="15104" --> < / p >
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< p > <EFBFBD> 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.< / p >
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