Functions » Common problems

We will discuss some common mistakes that beginners often make when using functions.

Incorrect number or type of arguments

In the following C++ code, the function sum expects two integer arguments, but the call to the function passes in a string and an integer:

int sum(int a, int b){
	return a + b;
}
int main(){
	std::string num = "5";
	std::cout << sum(num, 3);
}

The compiler will give an error:

error: no matching function for call to ‘sum(std::string&, int)’

To fix this mistake, pass in two integers to the function call like this:

int main(){
	std::cout << sum(5, 3);
}

Not returning a value

Consider the following example: the concatenate function expects to return a string, but the function does not return anything:

std::string concatenate(std::string a, std::string b)
{
	a += b;
} // no return statement at the end

The compiler will give an error:

error: control reaches end of non-void function [-Werror=return-type]

To fix this mistake, include a return statement in the function to return the concatenated string:

std::string concatenate(std::string a, std::string b) {
	a += b;
	return a;
}

tip

Note that the main function is an exception to this rule. It does not need to return a value. If the main function does not return a value, the compiler will assume that the function returns 0.

Missing a declaration

In the following code, the function multiply is called before it is declared:

int main(){
	std::cout << multiply(3, 4); // error: use of undeclared identifier ‘multiply’
}
int multiply(int a, int b){
	return a * b;
}