Function Declaration Vs Function Definition

A function declaration and a function definition are two important concepts in C++. A declaration tells the compiler that a function exists, while a definition provides the actual implementation of that function.

• A function declaration specifies what the function looks like.
• A function definition specifies how the function works.

Function Declaration

A function declaration (also known as a function prototype) informs the compiler about a function's name, return type, and parameters without providing its implementation. It allows the compiler to recognize the function before its actual definition appears in the program.

• Specifies the function's name, return type, and parameter list.
• Allows the function to be called before its definition in the source code.

Syntax

return_type function_name(parameter_list);

#include <iostream>
using namespace std;

// Function Declaration
int add(int, int);

int main()
{
    cout << "Sum = " << add(5, 3);

    return 0;
}

// Function Definition
int add(int a, int b)
{
    return a + b;
}

Sum = 8

Function Definition

A function definition provides the actual implementation of a function. It contains the executable statements that define what the function does when it is called and must match its declaration.

• Contains the function body, including the logic to be executed.
• Specifies the function's return type, name, and parameters, enabling the function to perform its intended task.

Syntax

return_type function_name(parameter_list){
// Function body
// Statements to be executed
}

#include <iostream>
using namespace std;

// Function Declaration
double calculateArea(double);

int main()
{
    double radius = 7;

    cout << "Area = " << calculateArea(radius);

    return 0;
}

// Function Definition
double calculateArea(double radius)
{
    return 3.14159 * radius * radius;
}

Area = 153.938

Difference between Function Declaration and Function Definition