C++ Class Methods

A class method (member function) is a function declared inside a class that operates on that class's objects, defined either inline within the class body or separately using the scope resolution operator.

In this page:

What is a Class Method?
Defining a Method Inline
Defining a Method Outside the Class
Methods that Return a Value
Methods that Take Parameters

Syntax

cpp

class ClassName {
public:
  return_type method_name(parameters) {
    // body
  }
};

ObjectName.method_name(arguments);

What is a Class Method?

A class method, also called a member function, is a function declared as part of a class, and every object of that class can call it, giving the method automatic access to that specific object's data.

Example: What is a Class Method?

cpp

// Include std::cout and std::cin
#include <iostream>

// Define the type Greeter
class Greeter {
// Members below can be accessed from outside the class
public:
	void greet() { std::cout << "Hello!" << std::endl; }
};

// Program execution starts in main()
int main() {
	Greeter g;
	g.greet();
	// Return 0 to signal that the program finished successfully
	return 0;
}

Defining a Method Inline

A method's body can be written directly inside the class definition, called an inline definition, which is convenient for short methods but keeps the class declaration longer.

Example: Defining a Method Inline

cpp

#include <iostream>

class Box {
public:
	int getSize() { return 10; } // defined inline, right inside the class
};

int main() {
	Box b;
	std::cout << b.getSize() << std::endl;
	return 0;
}

Defining a Method Outside the Class

A method can be declared inside the class but defined later outside it, using the class name followed by :: (the scope resolution operator) to indicate which class the definition belongs to.

Example: Defining a Method Outside the Class

cpp

// Include std::cout and std::cin
#include <iostream>

// Define the type Box
class Box {
// Members below can be accessed from outside the class
public:
	int getSize();
};

int Box::getSize() {
	// Send 20 back to the caller
	return 20;
}

// Program execution starts in main()
int main() {
	Box b;
	// Print to the console with cout
	std::cout << b.getSize() << std::endl;
	// Return 0 to signal that the program finished successfully
	return 0;
}

Methods that Return a Value

A method can return a value just like a regular function, computed from the calling object's own member variables, letting the object provide information about its own state.

Example: Methods that Return a Value

cpp

// Include std::cout and std::cin
#include <iostream>

// Define the type Circle
class Circle {
	// Declare radius and set it to 2.0
	double radius = 2.0;
// Members below can be accessed from outside the class
public:
	double area() { return 3.14159 * radius * radius; }
};

// Program execution starts in main()
int main() {
	Circle c;
	// Print to the console with cout
	std::cout << c.area() << std::endl;
	// Return 0 to signal that the program finished successfully
	return 0;
}

Methods that Take Parameters

A method can accept its own parameters in addition to having access to the object's member variables, letting it combine external input with the object's internal state.

Example: Methods that Take Parameters

cpp

// Include std::cout and std::cin
#include <iostream>

// Define the type Calculator
class Calculator {
// Members below can be accessed from outside the class
public:
	int add(int a, int b) { return a + b; }
};

// Program execution starts in main()
int main() {
	Calculator calc;
	// Print to the console with cout
	std::cout << calc.add(3, 4) << std::endl;
	// Return 0 to signal that the program finished successfully
	return 0;
}

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