Question

Given an abstract interface and an implementation derived from that interface, where constructors are protected (creation of these objects only being available from a class factory – to implement a DI pattern), how can I make use of make_shared in the factory function?

For example:

class IInterface
{    
public:    
    virtual void Method() = 0;
};

class InterfaceImpl : public IInterface
{
public:
    virtual void Method() {}

protected:    
    InterfaceImpl() {}    
};

std::shared_ptr<IInterface> Create()
{
    std::shared_ptr<IInterface> object = std:: make_shared<InterfaceImpl>();    
    return object;
}

make_shared obviously cannot access the protected constructor in InterfaceImpl, or indeed in IInterface, giving me the following error

error C2248: 'InterfaceImpl::InterfaceImpl' : cannot access protected member declared in class 'InterfaceImpl'

So reading here (question: How to make boost::make_shared a friend of my class), I tried putting the following into the implementation class:

friend std::shared_ptr<InterfaceImpl> std::make_shared<InterfaceImpl>();

It still wouldn’t compile. So then I put another one into the IInterface class too. Still no joy. What have I done wrong here?

EDIT: Full source file used to compile, with “friend”…

#include <memory>

class IInterface
{    
public:    
    friend std::shared_ptr&lt;IInterface> Create();     
    virtual void Method() = 0;
};

class InterfaceImpl : public IInterface
{    
public:     
    virtual void Method() {}

protected:    
    friend std::shared_ptr&lt;IInterface> Create();     
    InterfaceImpl() {}    
};

std::shared_ptr<IInterface> Create()
{
    std::shared_ptr<IInterface> object = std::make_shared<InterfaceImpl>();    
    return object;
}

void main()
{
    std::shared_ptr<IInterface> i = Create();   
}

Answer 1

With VC10 the solution you linked to doesn’t work – the construction of the instance of InterfaceImpl doesn’t happen in make_shared, but in an internal type in std::tr1::_Ref_count_obj<Ty>::_Ref_count_obj(void).

I’d just make the Create() function a friend in your case and not use make_shared():

class InterfaceImpl : public IInterface {
// ...    
protected:
    friend std::shared_ptr<IInterface> Create();
    InterfaceImpl() {}
};

std::shared_ptr<IInterface> Create() {
    return std::shared_ptr<IInterface>(new InterfaceImpl());
}

… or use a custom make_shared() implementation that you actually can befriend without relying on ugly implementation details.

An alternative would be to use something like this pass-key-idiom:

class InterfaceImpl : public IInterface {
public:
    class Key {
        friend std::shared_ptr<IInterface> Create();
        Key() {}
    };
    InterfaceImpl(const Key&) {}
};

std::shared_ptr<IInterface> Create() {
    std::shared_ptr<IInterface> object = 
        std::make_shared<InterfaceImpl>(InterfaceImpl::Key());
    return object;
}