Function declaration and definition in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple function call

What is the output of this C++ program?

C++

#include <iostream>

int add(int a, int b);

int main() {
    std::cout << add(3, 4) << std::endl;
    return 0;
}

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

A 7

B 34

C 0

D Compilation error

💡 Hint

Look at what the function 'add' returns when called with 3 and 4.

The function 'add' takes two integers and returns their sum. So add(3,4) returns 7, which is printed.

❓ Predict Output

intermediate

Function declaration without definition

What error will this C++ code produce when compiled?

C++

#include <iostream>

int multiply(int x, int y);

int main() {
    std::cout << multiply(2, 3) << std::endl;
    return 0;
}

A Linker error: undefined reference to 'multiply(int, int)'

B Runtime error

C Output: 6

D Compilation error: multiply not declared

💡 Hint

The function is declared but not defined anywhere.

The compiler knows the function signature but cannot find its body during linking, causing a linker error.

❓ Predict Output

advanced

Function overloading and output

What is the output of this C++ program?

C++

#include <iostream>

int compute(int x) {
    return x * 2;
}

double compute(double x) {
    return x / 2.0;
}

int main() {
    std::cout << compute(10) << " " << compute(10.0) << std::endl;
    return 0;
}

A 20 0

B Compilation error

C 10 5

D 20 5

💡 Hint

Look at which function is called for int and double arguments.

compute(10) calls the int version returning 20; compute(10.0) calls the double version returning 5.0, printed as 5.

🔧 Debug

advanced

Identify the error in function definition

What error does this code produce?

C++

#include <iostream>

void greet();

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

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

A Linker error: undefined reference to 'greet()'

B Syntax error: missing semicolon

C Runtime error

D No error, outputs 'Hello, world!'

💡 Hint

Check the last line of the function definition.

The function greet() is missing a semicolon after std::endl, causing a syntax error.

🧠 Conceptual

expert

Function declaration and definition order

Which statement is true about this code snippet?

C++

#include <iostream>

int square(int x) {
    return x * x;
}

int main() {
    std::cout << square(5) << std::endl;
    return 0;
}

A Function must be declared before main to compile

B Function cannot be called before its declaration or definition

C Function definition before main allows calling without prior declaration

D This code will cause a linker error

💡 Hint

Think about whether a function definition counts as a declaration.

Defining a function before main acts as its declaration, so main can call it without a separate declaration.

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