In OO parlance, the term method is used to denote a function within a class.

If a class 's methods were to become increasingly large it could make the code difficult to read, and it is therefore common practice to declare prototypes for the methods inline (i.e. within the class) and to then define the implementation of the methods outside of the class's body, to then be accessed using the :: scope resolution operator.

#include <iostream>
using namespace std ;

class Triangle {

private:
	float width ;
	float height ;

public:
	void setWidth(float) ;  //setter prototype
	void setHeight(float) ;
	float getWidth() ;      //getter prototype
	float getHeight() ;
	float area() ;          //method prototype

} ;

void Triangle::setWidth(float x){ //REMEMBER TO DECLARE RETURN TYPE
	width = x ;
}
void Triangle::setHeight(float y){ //REMEMBER TO DECLARE RETURN TYPE
	height = y ;
}
float Triangle::getWidth(){        //REMEMBER TO DECLARE RETURN TYPE
	return width ;
}
float Triangle::getHeight(){       //REMEMBER TO DECLARE RETURN TYPE
	return height ;
}
float Triangle::area(){            //REMEMBER TO DECLARE RETURN TYPE
	return ((width * height) / 2) ;
}

int main () {

	Triangle myTriangle ;

	myTriangle.setWidth(12) ;
	myTriangle.setHeight(2000) ;

	cout << "myTriangle width was set at " << myTriangle.getWidth() << endl ;
	cout << "myTriangle height was set at " << myTriangle.getHeight() << endl ;
	cout << "myTriangle area: " << myTriangle.area() << endl ;

	return 0;
}

Compile & Run: