使用模板调用类的函数

Question

I might be asking for something absurd here (and I am new with c++), but still I want to give it a try. I am trying to access a method of a class in a function that receives it's object, but, at the same time, I wanted to somehow not have to specify which class is that, that's why I am using templates. This sounds incredibly stupid, but anyway, that's the code:

class Dog;

class Animals{
public:
// initializes all protected variables
Animals(string fam, string gen, string espec, string subespec);
~Animals();

template <class A>
void Eat(A &);

protected:
    string family;
    string genre;
    string especies;
    string subespecies;
};

template <class A>
void Animals::Eat(A &obj)
{
    // this is where I don't know what I could do.
    std::cout << obj.methodOfA();
}

/---

int main()
{
    Dog *myDog = new Dog;
    myDog->Eat(myDog);

    //supposing I had class "Cats"
    Cat *myCat = new Cat;
    myCat->Eat(myCat);
}

This might be quite stupid, so feel free to point out my stupidity on the answers... if anyone ever answer this. I wanted this because I'll have many classes that are derived of class Animals, and independent of which of those classes, I wanted them to be able to use this method properly.

Thanks.

Answer 1

One way you could solve your problem is that you pass the function to be executed as a second argument. Then you can invoke the function with the operator.* or operator->* .

This could look something like this:

class Animals {
public:
// initializes all protected variables
Animals(std::string fam, std::string gen, std::string espec, std::string subespec);
~Animals();

template <class A, class Fun>
void Eat(A *&, Fun);

protected:
    std::string family;
    std::string genre;
    std::string especies;
    std::string subespecies;
};

template <class A, class Fun>
void Animals::Eat(A *& obj, Fun function)
{
    std::cout << ((obj->*function)()).c_str();
}


class Dog : public Animals {
public:
    std::string TestFunction() {
        return std::string("I am a dog");
    }
};


int main() {

    Dog *myDog = new Dog;
    myDog->Eat(myDog, &Dog::TestFunction);

    //...
}

The output of this program would be:

I am a dog

To clarify this line:

((obj->*function)())

You first apply the operator->* to obj and pass it the address of the member function function . Note that you have to put parantheses around this statement. After that you invoke it.( () ).

If there are any questions concerning the code provided above feel free to ask.

Regards,

Philinator