Why doesn't data member inherit in a diamond problem c++

Question

So, I was learning about inheritance and there was a question in last year's question set. It was just to create a diamond problem and inherit data member marks from base class to final class. So, I created a abstract base class for void setmarks() and override it in all inherited class. To resolve the ambiguity, I add virtual in inheritance declaration.

Here is my code:

#include <iostream>

using namespace std;

class Evalution

{

    public:
    float marks;

    virtual void setmarks()=0;
    
    void displaymarks() {}

};


class Theoretical :virtual protected Evalution
{
    public:
    float internal1, internal2;
    int assignments;
    
    void setmarks()
    {
        cout << "enter the marks in internal1, internal2, assignments" << endl;
        cin >> internal1 >> internal2 >> assignments;
    }
};


class Practical :virtual protected Evalution
{
    public:
    float lab, viva;
    
    void setmarks()
    {
        cout << "enter the marks in lab and viva" << endl;
        cin >> lab >> viva;
    }
};


class Final : public Theoretical, public Practical
{
    public:
     marks = internal1 + internal2 + assignments + lab + viva;
    
    void setmarks() {}
     void displaymarks()
    {
        cout << "the final marks is " << marks << endl;
    }
};

int main()
{
    Final f;
    f.Theoretical::setmarks();
    f.Practical::setmarks();
    f.displaymarks();
}

While running the program on online gcc compiler, there is error ‘marks’ does not name a type. What went wrong? Even if:

class Evalution
{
    public:
     //float marks;
     virtual void setmarks()=0;
     void displaymarks() {}
};

And:

class Final : public Theoretical, public Practical
{
    public:
    float marks = internal1 + internal2 + assignments + lab + viva;
    
    void setmarks() {}
     void displaymarks()
    {
        cout << "the final marks is " << marks << endl;
    }
};

The program still gives me a garbage value. Help please.

Answer 1

At this point, the compiler expects a member declaration

class Final : public Theoretical, public Practical {
public:
    marks = internal1 + internal2 + assignments + lab + viva;

Therefore it looks for a type. But marks is not a type, but a member variable in the base class. This is why the compiler complains about

'marks' does not name a type

---

When you change it to

float marks = ...;

the compiler sees the expected member declaration.

But this time the member marks is initialized at object construction time, not after the other members are assigned proper values.

---

For details, see Class declaration and Non-static data members, or maybe one of the books at The Definitive C++ Book Guide and List

Answer 2

You need to first set the variables internal1, internal2 etc. which you are doing through setmarks() base class methods before calculating final marks and then set final marks by calculating the sum of all.

#include <iostream>

using namespace std;

class Evalution

{

    public:
    float marks;

    virtual void setmarks()=0;
    
    void displaymarks() {}

};


class Theoretical :virtual protected Evalution
{
    public:
    float internal1, internal2;
    int assignments;
    
    void setmarks()
    {
        cout << "enter the marks in internal1, internal2, assignments" << endl;
        cin >> internal1 >> internal2 >> assignments;
        //internal1 = 10;
        //internal2 = 20;
        //assignments = 30;
    }
};


class Practical :virtual protected Evalution
{
    public:
    float lab, viva;
    
    void setmarks()
    {
        cout << "enter the marks in lab and viva" << endl;
        cin >> lab >> viva;
        //lab = 234;
        //viva = 45;
    }
};


class Final : public Theoretical, public Practical
{
    public:
    
    void setmarks() {
        this->Theoretical::setmarks();
        this->Practical::setmarks();
        marks = internal1 + internal2 + assignments + lab + viva;
    }
     void displaymarks()
    {
        cout << "the final marks is " << marks << endl;
    }
};

int main()
{
    Final f;
    f.setmarks();
    f.displaymarks();
}