Re: [boost] Interest in a protected pointer

Subject: Re: [boost] Interest in a protected pointer
From: Ray Logel (rlogel_at_[hidden])
Date: 2009-02-11 09:33:53

Dmitry Goncharov wrote:
> 
> 
> Ray Logel wrote:
>>
>> Is there any interest in pointer wrapper similar to scoped_ptr or 
>> shared_ptr which can be used for pointer member variable which can the 
>> referenced value can't be modified by non const functions?  This class 
>> wouldn't be responsible for any memory management and could have a 
>> shared pointer version as well.
>>
>> For example:
>> template<typename T>
>> class protected_ptr
>> {
>>     T * ptr;
>>
>>     // ...
>>     T * get();
>>     const T * get() const;
>>
>>     // ...  including operator* and operator->
>> }
>>
>> class Foo
>> {
>>     protected_ptr<Bar> _bar;
>>
>>     void Func1()
>>     {
>>         _bar->NonConstFunc();
>>         _bar->ConstFunc();
>>     }
>>
>>     void Func2() const
>>     {
>>         _bar->NonConstFunc();  // Compile error
>>         _bar->ConstFunc();     // Ok
>>     }
>> }
>>
>> Ray Logel
>>
>>
> The could be useful to propagate constness, however it looks like the 
> boost smart pointers deliberately dont do that.
> The following is an excerpt from shared_ptr's faq 
> (http://www.boost.org/doc/libs/1_38_0/libs/smart_ptr/shared_ptr.htm#functions): 

This class would have no effect on the current implementation of 
shared_ptr, which I agree handles constness the way they should.  It 
would only act as a protective wrapper around either a raw pointer or a 
shared_ptr when they are used as member variable and you would like to 
propagate constness within const member functions.

When I suggested there could be a shared_ptr version as well I meant 
something like this:

template<typename T>
class protected_ptr
{
   private:
     T * ptr;
     // ....
}

template<typename T>
class protected_shared_ptr
{
   private:
     shared_ptr<T> ptr;
     // ...
}

> 
> 
> "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"."
> 
> BR, Dmitry
> 
> 
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