My task is to practice inheritance, putting all the classes in separate files. I have a base class Circle and a derived class Cylinder.

What I'm stuck on is trying to display on the screen the result of my calculated area and volume for an object B of a Cylinder type. I found a way to do that for a Circle, though it doesn't work for my Cylinder.

circle.h

#pragma once
#include <iostream>

class Circle {
 public:
    float r; 
    Circle();
    Circle(float r);
    Circle circumference();
    Circle area();
    void view();
    void getArea();
    void getCircum();
};

circle.cpp

#include "circle.h"
#include <cmath>

using namespace std;

Circle::Circle() : r(5) {
    cout << "Default constructor has been called for a circle\n";
}

Circle::Circle(float r) {
    this->r = r;
    cout << "Constructor with parameters has been called for a circle\n";
}

void Circle::view() {
    cout << "Radius = " << r << endl;
}

Circle Circle::circumference() {
    return 2 * M_PI * r;
}

Circle Circle::area() {
    return M_PI * pow(r, 2);
}

void Circle::getArea() {
    cout << "Area = " << r << " m^2";
}

void Circle::getCircum() {
    cout << "Circumference = " << r << " m";
}

cylinder.h

#pragma once
#include <iostream>
#include "circle.h"

class Cylinder : public Circle {
 public:
    float h; 
    Cylinder();
    Cylinder(float r, float h);
    void view();
    double area();
    double volume(float r, float h);
    void getArea();
    void getVolume();
};

cylinder.cpp

#include "cylinder.h"
#include <cmath>

using namespace std;

Cylinder::Cylinder() : h(7) {
    cout << "Default constructor has been called for a cylinder\n";
}

Cylinder::Cylinder(float r, float h) : Circle(r) {
    this->h = h;
    cout << "Constructor with parameters has been called fo a cylinder\n";
}

void Cylinder::view() {
    Circle::view();
    cout << "Height = " << h << endl;
}

double Cylinder::area() {
    return 2 * M_PI * r * h;
}

double Cylinder::volume(float r, float h) {
    return M_PI * pow(r, 2) * h;
}

void Cylinder::getArea() {
    cout << "Area = " << h;
}

void Cylinder::getVolume() {
    cout << "Volume = " << h;
}

main.cpp

#include <iostream>
#include "circle.h"
#include "cylinder.h"

using namespace std;

int main() {
    Circle A;
    A.view();
    Circle A1(8);
    A1.view();
    
    Cylinder B;
    B.view();
    Cylinder B1(4, 6);
    B1.view();
    
    //A.area().getArea();
    //cout << endl;
        //A.circumference().getCircum();
        //cout << endl;
        //A1.area().getArea();
    //cout << endl;
        //A1.circumference().getCircum();
    
        B.area().getArea();

    return 0;
}

The error that I'm getting:

main.cpp: In function ‘int main()’:
main.cpp:26:14: error: request for member ‘getArea’ in ‘B.Cylinder::area()’, which is of non-class type ‘double’
   26 |     B.area().getArea();
      |              ^~~~~~~

I feel like neither my code in main() for instance B, nor my methods getArea() and getVolume() in class Cylinder, are correct. And there is probably a better approach to do the same for an object A and A1 of a Circle type, though the code I commented out actually works.

I know that this is a dumb question, and such things should be quite straightforward, but I am trying to learn and would be grateful for any advice on how I can fix this.

CodePudding user response：

Well, the reason you are getting the error message:

main.cpp: In function ‘int main()’:
main.cpp:26:14: error: request for member ‘getArea’ in ‘B.Cylinder::area()’, which is of non-class type ‘double’
   26 |     B.area().getArea();
      |              ^~~~~~~

Is because you are basically doing this:

auto _ = B.area();

So here, _ deduces to be a double, and then you do:

_.getArea();

You are trying to access a member function from a double, and double doesn't have any member functions.

You probably meant to do this instead:

auto x = B.area();
B.h = x;
B.GetArea();

This assigns the area of B.area() to a variable x, and assigns x to B.h. B's member function then gets called and outputs the area.

CodePudding user response：

In your getArea() function, instead of saying:

cout << "Area = " << endl;

Just say:

cout << "Area = " << area() << endl;

Then in your main.cpp, just call B.getArea().

Hope this helps!