C++ User-defined Function Types

For better understanding of arguments and return values in functions, user-defined functions can be categorized into various types:

• Function with no argument and no return value
• Function with no argument but return value
• Function with argument but no return value
• Function with argument and return value

Consider a situation in which you have to check prime number. Let's do that by making user-defined functions in 4 different ways as mentioned above.

No Arguments Passed and No Return Value

#include <iostream>
using namespace std;

// declare a function
// with no arguments and no return value
void check_prime();

int main() {
    // no argument is passed to prime()
    check_prime();
    return 0;
}

// function to check if the number is prime
void check_prime() {
    int num;
    
    cout << "Enter a positive integer to check: ";
    cin >> num;

    bool is_prime = true;;
    for(int i = 2; i <= num/2; ++i) {
        if(num % i == 0) {
            is_prime = false;; 
            break;
        }
    }

    if (is_prime) {
        cout << num << " is a prime number.";
    }
    else {
        cout << num << " is not a prime number.";
    }
}

Output

Enter a positive integer to check: 7
7 is a prime number.

Here,

• We haven't passed any argument to check_prime().
• The return type of check_prime() is void so it doesn't return any value.

The check_prime() function takes user input for a number, checks whether the given number is prime and gives output all by itself.

No Arguments Passed but a Return Value

#include <iostream>
using namespace std;

// declare a function with no arguments
// and return type int
bool check_prime();

int main() {

    // call prime and assign return value to is_prime
    bool is_prime = check_prime();
    
    if (is_prime) {
        cout << "Your number is a prime number.";
    }
    else {
        cout << "Your number is not a prime number.";
    }
    return 0;
}

// function to check if the number is prime
bool check_prime() {
    int num;
    
    cout << "Enter a positive integer to check: ";
    cin >> num;

    bool is_prime = true;
    for (int i = 2; i <= num / 2; ++i) {
        if (num % i == 0) {
            is_prime = false;
            break;
        }
    }
    
    // return the value
    return is_prime;
}

Output

Enter a positive integer to check: 8
Your number is not a prime number.

Here,

• We haven't passed any argument to check_prime().
• The return type of check_prime() is bool so it returns the is_prime variable of bool type.

The check_prime() function takes user input for a number, checks whether the given number is prime and returns is_prime.

The main() function gives the output based on the value returned by check_prime().

Arguments Passed but no Return Value

#include <iostream>
using namespace std;

// declare a function with int argument
// and no return value
void check_prime(int n);

int main() {
    cout << "Enter a positive integer to check: ";
    int num;
    cin >> num;
    
    // pass argument num function prime()
    check_prime(num);
    return 0;
}

// function to check if the number is prime
void check_prime(int n) {
    bool is_prime = true;
    for (int i = 2; i <= n/2; ++i) {
        if (n%i == 0) {
            is_prime = false;
            break;
        }
    }

    if (is_prime) {
        cout << n << " is a prime number.";
    }
    else {
        cout << n << " is not a prime number.";
    }
}

Output

Enter a positive integer to check: 37
37 is a prime number.

Here,

• We have passed one argument of int type to check_prime().
• The return type of check_prime() is void so it doesn't return any value.

The main() function calls check_prime() with argument num.

The check_prime() function checks whether the given number is prime and gives the output.

Arguments Passed and a Return Value

#include <iostream>
using namespace std;

// declare a function
// with int argument and int return type
bool check_prime(int n);

int main() {

    int num;
    cout << "Enter a positive integer to check: ";
    cin >> num;
    
    int is_prime = check_prime(num);

    if (is_prime == true)
        cout << num << " is a prime number.";
    else
        cout << num << " is not a prime number.";

    return 0;
}

// function to check if the number is prime
bool check_prime(int n) {
    
    for (int i = 2; i <= n/2; ++i) {
        if (n % i == 0)
            return false;
    }

    return true;
}

Output

Enter a positive integer to check: 14
14 is not a prime number.

Here,

• We have passed one argument of int type to check_prime().
• The return type of check_prime() is bool so it returns either true or false.

The main() function calls check_prime() with argument num.

The check_prime() returns true if the number is prime and false otherwise. Then main() gives the output based on the value returned by check_prime().

Also Read:

• C++ Functions
• C++ for loop