Auto ptr

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In the C++ programming language, auto_ptr is an obsolete smart pointer class template that was available in previous versions of the C++ standard library (declared in the <memory> header file), which provides some basic RAII features for C++ raw pointers. It has been replaced by the unique_ptr class.

Contents

The auto_ptr template class describes an object that stores a pointer to a single allocated object that ensures that the object to which it points gets destroyed automatically when control leaves a scope. [1]

The characteristics of auto_ptr are now considered unsatisfactory: it was introduced before C++11's move semantics, so it uses copying for what should be done with moves (and confusingly sets the copied-from auto_ptr to a null pointer). These copy semantics mean that it cannot be used in STL containers. [2]

The C++11 standard made auto_ptr deprecated, replacing it with the unique_ptr class template. [3] [4] auto_ptr was fully removed in C++17. [5] For shared ownership, the shared_ptr template class can be used. shared_ptr was defined in C++11 and is also available in the Boost library for use with previous C++ versions. [6]

Declaration

The auto_ptr class is declared in ISO/IEC 14882, section 20.4.5 as:

namespacestd{template<classY>structauto_ptr_ref{};template<classX>classauto_ptr{public:usingelement_type=X;// 20.4.5.1 construct/copy/destroy:explicitauto_ptr(X*p=0)throw();auto_ptr(auto_ptr&)throw();template<classY>auto_ptr(auto_ptr<Y>&)throw();auto_ptr&operator=(auto_ptr&)throw();template<classY>auto_ptr&operator=(auto_ptr<Y>&)throw();auto_ptr&operator=(auto_ptr_ref<X>)throw();~auto_ptr()throw();// 20.4.5.2 members:X&operator*()constthrow();X*operator->()constthrow();X*get()constthrow();X*release()throw();voidreset(X*p=0)throw();// 20.4.5.3 conversions:auto_ptr(auto_ptr_ref<X>)throw();template<classY>operatorauto_ptr_ref<Y>()throw();template<classY>operatorauto_ptr<Y>()throw();};}

Semantics

The auto_ptr has semantics of strict ownership, meaning that the auto_ptr instance is the sole entity responsible for the object's lifetime. If an auto_ptr is copied, the source loses the reference. For example:

#include<iostream>#include<memory>usingstd::auto_ptr;intmain(intargc,char*argv[]){int*a=newint[10];auto_ptr<int[]>x(a);auto_ptr<int[]>y;y=x;std::cout<<x.get()<<std::endl;// Print NULLstd::cout<<y.get()<<std::endl;// Print non-NULL address of areturn0;}

This code will print a NULL address for the first auto_ptr object and some non-NULL address for the second, showing that the source object lost the reference during the assignment (=). The raw pointer i in the example should not be deleted, as it will be deleted by the auto_ptr that owns the reference. In fact, new int could be passed directly into x, eliminating the need for i.

Notice that the object pointed by an auto_ptr is destroyed using operator delete; this means that you should only use auto_ptr for pointers obtained with operator new. This excludes pointers returned by malloc/calloc/realloc , and pointers to arrays (because arrays are allocated by operator new[] and must be deallocated by operator delete[]).

Because of its copy semantics, auto_ptr may not be used in STL containers that may perform element copies in their operations.

See also

References

  1. ↑ "auto_ptr Class". Microsoft. Retrieved 2006-09-27.
  2. ↑ Meyers, Scott (2014). Effective Modern C++: 42 specific ways to improve your use of C++11 and C++14. Sebastopol, CA. p. 118. ISBN   978-1-491-90399-5.{{cite book}}: CS1 maint: location missing publisher (link)
  3. ↑ "Working Draft, Standard for Programming Language C++ N3242" (PDF). 28 February 2011. p. 1233. Retrieved 2013-02-17.
  4. ↑ Kalev, Danny. "Using unique_ptr, Part I". informIT. Retrieved 30 September 2010.
  5. ↑ "Programming Language C++, Library Evolution Working Group JTC1/SC22/WG21 N4190". 2014-10-09. Retrieved 2016-08-29.
  6. ↑ "Collecting Shared Objects". Dr. Dobb's. 2004-07-01. Retrieved 2006-09-27.
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