Re: [Boost-users] Conversions between shared_ptr<Base> and shared_ptr<Derived>

From: Duane Murphy (duanemurphy_at_[hidden])
Date: 2004-02-04 10:29:55

--- At Wed, 4 Feb 2004 15:29:50 +0200, Peter Dimov wrote:

>Joshua Little wrote:
>[...]
>>
>> So I have something like this:
>>
>>  std::vector<boost::shared_ptr<Base> > BaseVector;
>>
>> void add(boost::shared_ptr<Base>& newOne) {
>>     BaseVector.push_back(newOne);
>> }
>>
>> and I call it like this usually :
>> boost::shared_ptr<Derived> p(new Derived);
>> add(p);
>>
>> I thought that it would work just like using raw pointers (which was
>> working) but I get a compile time error that :
>> no matching function call to add(boost::shared_ptr<Derived>&)
>>     canadites are add(boost::shared_ptr<Base>&).. ect.
>
>It should fail with raw pointers, too, unless your compiler is too
>permissive. The problem is that
>
>    void add(boost::shared_ptr<Base>& newOne)
>
>takes a non-const reference to shared_ptr<Base>, so you need a
>shared_ptr<Base> to call it. Change it to
>
>    void add(boost::shared_ptr<Base> const & newOne)
>
>and it should work. And, as a general rule, always use a reference to const
>when you don't intend to modify the argument.

Peter is quite correct with the change to the argument type. It seems to
me the solution could be had another way:

boost::shared_ptr<Base> p(new Derived);
                  ----
add(p);

Then p is the correct type. The value being added to the vector will be a
Base and not a Derived, so you loose nothing by using the shared_ptr as a
Base. The only catch would be using Derived "features" between the
construction of the shared pointer and the addition to the vector.

 ...Duane