Why do we need a post and pre operator overload?

[Tduck]

Why do we need a post increment and pre increment operator overload?

In the following code, written by the designers of the course, there are four operator overloads for incrementation and decrementation, ++ and --. They have a post increment and a pre increment one there, and I don't understand what they are for. Why is there a post and pre overload for the operator?

#ifndef H_rectangleType
#define H_rectangleType
  
#include <iostream>
using namespace std;

class rectangleType
{
        //Overload the stream insertion and extraction operators
    friend ostream& operator<<(ostream&, const rectangleType &);
    friend istream& operator>>(istream&, rectangleType &);

public:
    void setDimension(double l, double w);
    double getLength() const;
    double getWidth() const;
    double area() const;
    double perimeter() const;

    //Overload the arithmetic operators (part b)
    rectangleType operator + (const rectangleType &) const;
    rectangleType operator - (const rectangleType &) const;
    rectangleType operator * (const rectangleType&) const;

    //Overload the increment and decrement operators (part a)
    rectangleType operator ++ ();          //pre-increment
    rectangleType operator ++ (int);       //post-increment
    rectangleType operator -- ();          //pre-decrement
    rectangleType operator -- (int);       //post-decrement

    //Overload the relational operators (part c)
    bool operator == (const rectangleType&) const;
    bool operator != (const rectangleType&) const;
    bool operator <= (const rectangleType&) const;
    bool operator < (const rectangleType&) const;
    bool operator >= (const rectangleType&) const;
    bool operator > (const rectangleType&) const;

        //constructors
    rectangleType();
    rectangleType(double l, double w);

protected:
    double length;
    double width;
};
#endif 

[MikeyBoy]

The following two operations use different operators, because they do different things:

++i;
i++;

You need one operator to perform the first operation, and a different operator to do the second.

[Tduck]

Followup, how would I actually do that?

#include<iostream>
using namespace std;

struct GRAPH
{
    int x = 0;
    int y = 0;
    int operator++()
    {
        x = x+1;
        y = y+1;
        return->this;
    }
};


int main()
{
    GRAPH graph1;
    graph1++;
    
    return 0;
}

I wrote an example program to try to figure it out but don't know what I'm doing or what exactly "this" is capable of doing.