Why is my code is not working as expected?

Question

In the output, there was an error when I attempted to display it. The output is displayed without any errors, but the total marks and percentages are not displayed. I would appreciate if anyone could provide an explanation.

1. create a class student that stores roll_no,name.
2. create a class test that stores marks obtained in five subjects.
3. class result derived from student and test contains the total marks and percentage obtained in the test.
4. input and display information of a student.

#include <iostream>
using namespace std;
class student {
    int roll_no;
    string name;

public:
    void studentdata()
    {
        cout << "\n ENTER STUDENT NAME:";
        cin >> name;
        cout << "\nENTER ROLL NUMBER:";
        cin >> roll_no;
    }
};
class test {
    int i;
    // int marks[5{}];
public:
    int marks[5];
    void getmarks()
    {
        cout << "\nENTER MARKS OF EACH SUBJECT:";
        for (i = 0; i < 5; i++) {
            cout << "\n subject " << (i + 1) << " :";
            cin >> marks[i];
        }
    }
};
class result : public student, public test {
    int i;
    float totalmarks = 0;
    float percent;

public:
    void percentage()
    {
        // int total_marks=0;
        for (i = 0; i < 5; i++) {
            totalmarks += totalmarks + marks[i];
        }
        cout << " \n TOTAL MARKS OUT OF 500 IS:" << totalmarks;
        percent = (totalmarks * 100) / 500;
        cout << " \n \nPERCENTAGE OBTAINED IN THE TEST:" << percent << " %";
    }
};
int main()
{
    student s;
    test t;
    result r;
    s.studentdata();
    t.getmarks();
    r.percentage();
    return 0;
}

Answer 1

From your main function:

student s;
test t;
result r;
s.studentdata();
t.getmarks();
r.percentage();

You obviously missed the fact that a result instance, due to inheritance, already contains both a student and a test instance.

You now initialise separate student and test instances while those of your result instance r remain uninitialised.

To get meaningful results you need to initialise the instances of r – and as you don't use s or t any further you can drop both of them, what then remains is:

result r;
r.studentdata(); // r has inherited the function from student class!
r.getmarks();    // r has inherited the function from test class!
r.percentage();  // result's own function

Note, though, that this design, albeit mandated by your task, is exceptionally bad for several reasons; there is absolutely no reason why this result class should inherit from student and test, they are unrelated one from another, if at all result should aggregate the other two. Furthermore totalmarks and percent variables have no purpose for the result class, they are exclusively used within percentage function and thus should be local variables there – I know it's mandated by the task, but you should at least be aware of. But at very least you can make the loop counters (int i within both test and result classes) local variables of their respective functions: for(int i = 0; i < std::size(marks); ++i) – or even simpler, use range based for loops:

for (auto& m : marks)
//       ^ important here!
{
    std::cin >> m; // here you need the reference above!
}

for (auto m : marks)
{
    totalmarks += m;
}

The second loop doesn't need the reference and as it's a small type, it's irrelevant if you place it – but you might get used to principally adding it, by value is more efficient the compiler will optimise accordingly, though you can support it in its job by having a const reference:

for (auto const& m : marks)
{
    totalmarks += m;
}