---
title: 6.2 Function Declaration
description: "A function declaration, also called a function prototype, informs the compiler about a function before the function is called or defined. It specifies the function's name, return type, and parameter types. A function can therefore be declared before `main()` and…"
url: "https://edcret.in/docs/cpp-programming/functions/6-2-function-declaration/"
updated: "2026-09-06"
category: C++ Programming / Functions
---

# 6.2 Function Declaration

A function declaration, also called a function prototype, informs the compiler about a function before the function is called or defined. It specifies the function's name, return type, and parameter types.

A function can therefore be declared before `main()` and defined later in the source file. This allows the compiler to verify function calls even when the complete function definition has not yet been encountered.

## Source Code

```
#include <iostream>

int add(int, int);

int main()
{
    int result = add(10, 20);

    std::cout << "Sum: " << result << 'n';

    return 0;
}

int add(int a, int b)
{
    return a + b;
}
```

## Output

```
Sum: 30
```

## Explanation

- `int add(int, int);` is a function declaration.
- The declaration specifies that `add` returns an `int` and accepts two `int` parameters.
- Parameter names are optional in a function declaration, so only the parameter types are specified.
- The semicolon at the end indicates that this is a declaration rather than a function definition.
- `main()` can call `add()` because the compiler has already encountered its declaration.
- `add(10, 20)` calls the function with two integer arguments.
- The function definition appears after `main()`.
- `int add(int a, int b)` provides the function's implementation and names its parameters.
- `return a + b;` returns the calculated sum to the caller.
- The declaration and definition must have compatible return types and parameter types.