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.