Nested Classes variable calls - c++

Question

I want to go from this:
To this:
How would I do this? How would the functions of subclasses square and rectangle know to use the variable of the parent class shape?
How would I set length and width from main?
#include <iostream>
#include <cmath>
using namespace std;
class SHAPES
{
public:
class SQUARE
{
int perimeter(int length, int width)
{
return 4*length;
}
int area(int length, int width)
{
return length*length;
}
};
public:
class RECTANGLE
{
int perimeter(int length, int width)
{
return 2*length + 2*width;
}
int area(int length, int width)
{
return length*width;
}
};
};

Answer 1

I recommend other (better?!) format:
class Shape
{
protected:
int length,width;
public:
Shape(int l, int w): length(l), width(w){}
int primeter() const
{
return (length + width) * 2;
}
int area() const
{
return length * width;
}
};
class Rectangle : public Shape
{
public
Rectangle(int l, int w) : Shape(l,w){}
};
class Square : public Shape
{
public:
Square(int l): Shape(l,l){}
};
int main()
{
Rectangle r(5,4);
Square s(6);
r.area();
s.area();
}
Or use interface with virtual function.

Answer 2

Those are not subclasses (i.e. derived classes), but rather nested classes (as the title of your question says).
I don't think I would answer your real question if I were to tell you how to make those variables visible in nested classes. Based on what I can understand from the names of your classes, you should rather use inheritance to model the IS-A relation between them:
class SHAPE
{
public: // <-- To make the class constructor visible
SHAPE(int l, int w) : length(l), width(w) { } // <-- Class constructor
...
protected: // <-- To make sure these variables are visible to subclasses
int length;
int width;
};
class SQUARE : public SHAPE // <-- To declare public inheritance
{
public:
SQUARE(int l) : SHAPE(l, l) { } // <-- Forward arguments to base constructor
int perimeter() const // <-- I would also add the const qualifier
{
return 4 * length;
}
...
};
class RECTANGLE : public SHAPE
{
// Similarly here...
};
int main()
{
SQUARE s(5);
cout << s.perimeter();
}