Like the title says; Why would you want to make class objects (instances of a class) instead of built in types parameters of a function and in C . E.g. Why do this: class Person { public: void SetAge(Person &Age); instead of this: class Person { public: void SetAge(int iAge);. I don't really understand why don't you create parameters using only built in data types.

To pass information, and maybe change values in passed parameters where changes are retained outside the current function.

class Address // Assumes address is in USA
    std::string street;
    std::string city;
    std::string state;
    std::string zipcode;
    const std::string& getStreet() const;
    const std::string& getCity() const;
    const std::string& getState() const;
    const std::string& getZipCode() const;

class Map
    void showLocation( const Address& addr ) const;
    // Alternative implementation:
    void showLocation( const std::string& street,
                       const std::string& city,
                       const std::string& state,
                       const std::string& zipcode ) const;

So, the class Map can use the getter methods in Address::showLocation( const Address& ) const to determine where the location is, and then display that. Note that this is a VERY rudementary example, but the point is that passing objects instead of lower-level scalar data can be useful, more terse, and less error prone.

commented: Great answer! +15

This maybe can help you:

// classes example
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
    void set_values (int,int);
    int area() {return width*height;}

void Rectangle::set_values (int x, int y) {
  width = x;
  height = y;

int main () {
  Rectangle rect;
  rect.set_values (3,4);
  cout << "area: " << rect.area();
  return 0;
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