CODEWITHSUNDEEP
c++oopinheritancevectorstd
user2211216
user2211216

Reputation: 375

c++ vector pointer to base class, and access to multiple derived classes methods

class A{
public:
    int var;

    virtual int getVar() { return var; }
 };

class B: public A{
public:
    int anothervar;

    int getAnotherVar() { return anothervar; }
 };


class C: public A{
public:
    int finalvar;

    int getFinalVar() { return finalvar;}
 };

int main () {
    vector<A*> myvec;

    myvec.push_back (new B());  // implying all constructors are ok
    myvec.push_back (new C());
    cout << myvec[0]->geVar();         // this works fine
    cout << myvec[0]->getAnotherVar(); // how can I do this ?
    cout << myvec[1]->getFinalVar();   // how can I do this ?
    return 0;
}

This is just a representation of another problem I'm trying to solve. So my question is, if it is possible to call derived classes methods, from a pointer vector of a base class since I cannot declare them as pure virtual at the base, because I don't want to use them in both derived.

Upvotes: 0

Views: 2402

Answers (3)

Sivabushan
Sivabushan

Reputation: 71

you need to do casting here.

cout<<static_cast<B*>(myvec[0])->getAnotherVar(); //how can i do this ?
cout<<static_cast<C*>(myvec[1])->getFinalVar();  //how can i do this ?

C++ Access derived class member from base class pointer

Upvotes: 0

Martin J.
Martin J.

Reputation: 5118

You can do this by casting to the right type:

vector<A*>myvec; //pointer vector to base class
myvec.push_back(new B());  //implying all constructors are ok
myvec.push_back(new C());

cout << myvec[0]->geVar();  //this works fine
cout << myvec[1]->geVar();  //this works fine

B* b = dynamic_cast<B*>(myvec[0]);
assert(nullptr != b);
cout << b->getAnotherVar();

C* c = dynamic_cast<C*>(myvec[1]);
assert(nullptr != c);
cout << c->getFinalVar();

However, needing to access your collection in this kind of non-uniform manner may point to a bigger class design issue.
More precisely, here it looks all fine and dandy because you create the B and C class instances in the same place where you access them, so the implicit information of which class is at which index is "local", but in a realistic example, you would probably create the objects and access them in different places, and relying on that implicit index information for the non-uniform access you want will probably be a source of bugs later.

Upvotes: 3

Hulk
Hulk

Reputation: 6583

You can only invoke methods that the type declares. If you have a base pointer to a derived object and want to invoke a method that only the derived type declares, you need to cast to that type.

If you can not be sure at compile time what the type of the object is, you need to dynamic_cast which performs a runtime check and throws an exception in case of failure.

If you can guarantee the type at compile time, static_cast is the appropriate tool.

Upvotes: 0

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