Re: std::auto_ptr and const correctness

Maxim Yegorushkin <>
Tue, 12 May 2009 09:42:50 CST
On May 12, 2:26 pm, wrote:

in the project I am working on, std::auto_ptr is used to implement
move semantics and sometimes to store pointers in classes which cannot
be copied. When I analyzed some old code, I found a flaw in the
definition of std:.auto_ptr which breaks const-correctness of the
#include <memory>

class X
   void f() {i=42;}

   int i;


template<typename T>
class AutoPtr
   explicit AutoPtr(T* ptr) : t(ptr) {}

   T* operator->() {return t;}
   T const* operator->() const {return t;}

   T* get() {return t;}
   T const* get() const {return t;}

   T* t;


class Test
   void f() const
      X* x = autoPtr.get();

   void g() const
      X* x = customAutoPtr.get();

   std::auto_ptr<X> autoPtr;
   AutoPtr<X> customAutoPtr;


When this is compiled, the compiler only reports an error for method
Test::g(), not for Test::f(), e.g. the Comeau compiler:
Thank you for testing your code with Comeau C/C++!
Tell others about!

Your Comeau C/C++ test results are as follows:

Comeau C/C++ (Oct 6 2008 11:28:09) for
Copyright 1988-2008 Comeau Computing. All rights reserved.
MODE:strict errors C++ noC++0x_extensions

"ComeauTest.c", line 39: error: the object has cv-qualifiers that are
not compatible
          with the member function
            object type is: const X

"ComeauTest.c", line 40: error: a value of type "const X *" cannot be
used to
          initialize an entity of type "X *"
        X* x = customAutoPtr.get();

2 errors detected in the compilation of "ComeauTest.c".

Additionally, the standards specifies std::auto_ptr to have the
following members:
// members:
X& operator*() const throw();
X* operator>()
const throw();
X* get() const throw();
X* release() throw();
void reset(X* p =0) throw();

Is there any reason why the methods are specified const but return a
non-const pointer? I know that std::auto_ptr is deprecated, but this
looks as a serious flaw.

There is a general C++ rule for overloading operators: when in doubt,
do what the operator does for built-in types. std::auto_ptr operator->
and * behave exactly as they do for plain pointers: the const-ness of
a pointer does not affect the const-ness of the type of the value
being pointed to, i.e.:

    int* p1; // pointer to non-constant int
    int* const p2; // constant pointer to non-constant int
    int& r1 = *p1;
    int& r2 = *p2; // fine, it is p2 what is constant, not *p2

The same is true for std::auto_ptr<>:

    std::auto_ptr<int> p1; // auto-pointer to non-constant int
    std::auto_ptr<int> const p2; // constant auto-pointer to non-
constant int
    int& r1 = *p1;
    int& r2 = *p2; // fine, it is p2 what is constant, not *p2


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