Function declaration and definition in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function declaration in C++?

A function declaration tells the compiler about a function's name, return type, and parameters without providing the body. It acts like a promise that the function will be defined later.

beginner

What is a function definition in C++?

A function definition provides the actual body of the function. It includes the code that runs when the function is called.

intermediate

Why do we separate function declaration and definition?

Separating declaration and definition helps organize code. Declarations can be in header files so other files know about the function, while definitions are in source files with the actual code.

beginner

Example: Write a function declaration for a function named add that takes two integers and returns an integer.

int add(int a, int b);

beginner

Example: Write a function definition for add that returns the sum of two integers.

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

What does a function declaration NOT include?

A The function's body (code inside)

B The function's name

C The function's return type

D The function's parameters

A function declaration includes the name, return type, and parameters but does not include the body.

Where is a function definition usually placed?

A In a header (.h) file only

B Inside main() function

C In a source (.cpp) file

D In a comment block

Function definitions are usually placed in source files (.cpp) where the actual code is written.

What keyword is used to declare a function that returns no value?

A empty

B void

C null

D none

The keyword 'void' is used to declare functions that do not return any value.

Which of the following is a correct function declaration?

A int multiply x, y;

B int multiply(int x, int y) { return x * y; }

C multiply(int x, int y);

D int multiply(int x, int y);

Option D is a correct function declaration with return type, name, and parameters.

What happens if you call a function that is declared but not defined?

A Linker error occurs

B Program runs normally

C Compiler error occurs

D Function runs with empty body

If a function is declared but not defined, the linker cannot find the function's code and throws an error.

Explain the difference between a function declaration and a function definition in C++.

Think about what each part tells the compiler and where the code lives.

Write a simple function declaration and definition for a function that prints "Hello" to the screen.

Use void for no return and cout for printing.

Practice

1. What is the main purpose of a function declaration in C++?

easy

A. To execute the function's code

B. To tell the program the function's name, return type, and parameters

C. To create a variable inside the function

D. To print output to the screen

Solution

1. Step 1: Understand function declaration role

   A function declaration informs the compiler about the function's name, return type, and parameters without providing the actual code.

2. Step 2: Differentiate declaration from definition

   The function definition contains the actual code to run, while declaration only tells the program what to expect.

3. Final Answer:

   To tell the program the function's name, return type, and parameters -> Option B

4. Quick Check:

   Function declaration = function signature [OK]

Hint: Declaration = function signature only, no code [OK]

Common Mistakes:

• Confusing declaration with definition
• Thinking declaration runs code
• Mixing variable creation with function declaration

2. Which of the following is a correct function declaration in C++?

easy

A. int add(int a, int b);

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

C. int add = (int a, int b);

D. add(int a, int b) int;

Solution

1. Step 1: Identify correct syntax for declaration

   A function declaration includes the return type, function name, parameter list, and ends with a semicolon without a body.

2. Step 2: Check each option

   int add(int a, int b); matches declaration syntax. int add(int a, int b) { return a + b; } is a definition. Options C and D have invalid syntax.

3. Final Answer:

   int add(int a, int b); -> Option A

4. Quick Check:

   Declaration ends with semicolon, no body [OK]

Hint: Declaration ends with semicolon, no braces [OK]

Common Mistakes:

• Writing function body in declaration
• Missing semicolon at end
• Incorrect parameter syntax

3. What will be the output of this code?

int multiply(int x, int y);

int main() {
    int result = multiply(3, 4);
    std::cout << result;
    return 0;
}

int multiply(int x, int y) {
    return x * y;
}

medium

A. 0

B. 7

C. Error: multiply not defined

D. 12

Solution

1. Step 1: Understand function declaration and definition

   The function multiply is declared before main and defined after main, which is allowed in C++.

2. Step 2: Calculate multiply(3, 4)

   The function returns 3 * 4 = 12, which is printed by std::cout.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Declaration before use, definition after is valid [OK]

Common Mistakes:

• Thinking definition must be before main
• Confusing declaration with definition
• Expecting compile error without definition before main

4. Identify the error in this code:

void greet();

int main() {
    greet();
    return 0;
}

void greet() {
    std::cout << "Hello!" << std::endl
}

medium

A. Return type of greet should be int

B. Function greet is not declared

C. Missing semicolon after std::endl

D. Missing parentheses in greet call

Solution

1. Step 1: Check function declaration and definition

   The function greet is declared and defined correctly with void return type and no parameters.

2. Step 2: Identify syntax error in function body

   The line with std::cout << "Hello!" << std::endl is missing a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after std::endl -> Option C

4. Quick Check:

   Statements must end with semicolon [OK]

Hint: Check semicolons after output statements [OK]

Common Mistakes:

• Forgetting semicolon after std::endl
• Thinking declaration missing causes error
• Assuming wrong return type

5. You want to declare a function int max(int a, int b); and define it later. Which of these definitions correctly implements it to return the larger number?

hard

A. int max(int a, int b) { if (a > b) return a; else return b; }

B. int max(int a, int b) { return a + b; }

C. int max(int a, int b) { if (a < b) return a; else return b; }

D. int max(int a, int b) { return 0; }

Solution

1. Step 1: Understand function purpose

   The function max should return the larger of the two integers a and b.

2. Step 2: Evaluate each definition

   int max(int a, int b) { if (a > b) return a; else return b; } returns a if a > b, else b, correctly returning the maximum. Options A, B, and C do not correctly return the maximum value.

3. Final Answer:

   int max(int a, int b) { if (a > b) return a; else return b; } -> Option A

4. Quick Check:

   Return larger value using if-else [OK]

Hint: Use if (a > b) return a; else return b; [OK]

Common Mistakes:

• Returning sum instead of max
• Returning smaller value by mistake
• Returning constant zero