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A member function of a class is a function that has its definition or its prototype within the class definition like any other variable. It operates on any object of the class of which it is a member, and has access to all the members of a class for that object.

Let us take previously defined class to access the members of the class using a member function instead of directly accessing them −

class Box {
   public:
      double length;         // Length of a box
      double breadth;        // Breadth of a box
      double height;         // Height of a box
      double getVolume(void);// Returns box volume
};

Defining Class Member Functions

Member functions can be defined within the class definition or separately using scope resolution operator, : −. Defining a member function within the class definition declares the function inline, even if you do not use the inline specifier. So either you can define Volume() function as below −

Defining Member Function inside the Class

class Box {
   public:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
   
      double getVolume(void) {
         return length * breadth * height;
      }
};

Defining Member Function outside of the Class

If you like, you can define the same function outside the class using the scope resolution operator (::) as follows −

double Box::getVolume(void) {
   return length * breadth * height;
}

Here, only important point is that you would have to use class name just before :: operator.

Calling (Accessing) Member Functions

A member function will be called using a dot operator (.) on a object where it will manipulate data related to that object only as follows −

Box myBox;          // Create an object

myBox.getVolume();  // Call member function for the object

Example

Let us put above concepts to set and get the value of different class members in a class −

#include <iostream>

using namespace std;

class Box {
   public:
      double length;         // Length of a box
      double breadth;        // Breadth of a box
      double height;         // Height of a box

      // Member functions declaration
      double getVolume(void);
      void setLength( double len );
      void setBreadth( double bre );
      void setHeight( double hei );
};

// Member functions definitions
double Box::getVolume(void) {
   return length * breadth * height;
}

void Box::setLength( double len ) {
   length = len;
}
void Box::setBreadth( double bre ) {
   breadth = bre;
}
void Box::setHeight( double hei ) {
   height = hei;
}

// Main function for the program
int main() {
   Box Box1;                // Declare Box1 of type Box
   Box Box2;                // Declare Box2 of type Box
   double volume = 0.0;     // Store the volume of a box here
 
   // box 1 specification
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);

   // box 2 specification
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0);

   // volume of box 1
   volume = Box1.getVolume();
   cout << "Volume of Box1 : " << volume <<endl;

   // volume of box 2
   volume = Box2.getVolume();
   cout << "Volume of Box2 : " << volume <<endl;
   return 0;
}

When the above code is compiled and executed, it produces the following result −

Volume of Box1 : 210
Volume of Box2 : 1560