How to use shared_ptr on a 3'd party class with protected destructor and public destroy method

Question

I am a client of a class A which it's destructor is defined in protected. In addition I cannot change it's interface (I deliberately wrote "3'd party class", although I meant that for any reason, you aren't allowed to change its interface. So, How can I use boost::shared_ptr in such case? The destructor is virtual:

Class Foo {
    public:
        void Destroy () {}
    protected:
    virtual ~Foo () {}
}

For Foo, it provide a "Destroy" method

A Straight forward usage The following code won't compile:

  boost::shared_ptr <Foo> a = boost::make_shared <Foo> ();

Compiler message: ... error ..."Foo::~Foo() is inaccessible ...

Also, my workplace's compiler does not support c++11

Answer 1

You can create an intermediate class as a helper polymorphic base:

Sample using only c++03 Live On Coliru:

#include <boost/shared_ptr.hpp>
#include <iostream>

class Foo { // "abstract"
    public:
        virtual void Destroy () { std::cout << __FUNCTION__ << "\n"; }
    protected:
        virtual ~Foo () {}
};

class FooBase : public Foo {
    public:
        static void Deleter(FooBase* p) {
            if (p)
                p->Destroy();
            delete p;
        }

    // protected:
        virtual ~FooBase() { std::cout << __FUNCTION__ << "\n"; }
};

class FooDerived : public FooBase
{
    ~FooDerived() { std::cout << __FUNCTION__ << "\n"; }
};

int main()
{
    boost::shared_ptr<FooBase> p(new FooDerived, FooBase::Deleter);
}

Prints:

Destroy
~FooDerived
~FooBase

Note How the destructor could actually be protected now. That ensures that all destructions go through FooBase::Deleter