static_cast an interface to derived class

Question

I am trying to static_cast an interface object into object of derived class which inherits that interface. I am getting an error

'static_cast' : cannot convert from 'IInherit *' to 'cDerived *'

The derived class and interface are of the following format.

class cDerived: public IInherit
{
    Repo* p_Repos;
public:
    cDerived(Repo* pRepos)
    {
        p_Repos = pRepos;
    }
    Repo* GetRepo()
    {
            return p_Repos;
    }
    void doAction(ITok*& pTc)
    {
       ///some logic
    }

}

class IInherit
{
public:
    virtual ~IInherit() {}
    virtual void doAction(ITok*& pTc)=0;
};

I have a vector<IInherit*> object accessible in the code through getInherit() method such that the type of getInherit()[0] is cDerived* I am performing static cast using the expression:

Repo* pRep= static_cast<cDerived*>(getInherit()[0])->GetRepo();

I am not sure if it is possible to static_cast as interface object. Is there any other way that I would be able to perform this cast?

Solution

You can use static_cast in your example.

But you must include both definitions of IInherit and cDerived for it to work. The compiler must see, that cDerived inherits from IInherit. Otherwise, it cannot decide that the static_cast is indeed valid.

#include <vector>

struct R {};
struct B {};
struct D : public B {
    R *getR() { return new R(); }
};

void f()
{
    std::vector<B*> v;
    v.push_back(new D());
    D *d = static_cast<D*>(v[0]);
    R *r = d->getR();
}

Comments

[Lily Ballard]

What compiler are you using? This works for me.

[JoergB]

Can you provide a complete example showing the error and the full error message?

[Pete Becker]

Are the definitions of IInherit and cDerived both visible at the point where the error occurs?

[Saurav Seth]

By chance, is the real problem because you declared IInherit in your header file AFTER you declared cDerived as you did above? Or is that just how you cut&paste in the code above. There's a missing semi-colon after the delclaration of cDerived. I'm just wondering if compiler is erroring out on some other error before showing this error.

[vaibhav kumar]

No I have forward declared the derived class, no issues with visibility

[vaibhav kumar]

@KevinBallard vc++ compiler

[Peter Wood]

Why do you need to call GetRepo()?

[François Moisan]

This is the answer. static_cast requires more than a forward declaration to ensure the cast is at least possible (though you could still be lying to the compiler).

[Olaf Dietsche]

@vaibhavkumar Visibility alone is not the issue, the compiler must see the definition, i.e. the inheritance. I updated my answer.