#016 – Using function calls as arguments

Consider this program

#include <iostream>

double getRadius()
{
    std::cout << "Enter the radius: ";
    double radius{};
    std::cin >> radius;

    return radius;
}

void calculateArea(double r)
{
    std::cout << "The area of the circle is "
    << 3.14159 * r * r;
}

int main()
{
	double x = GetRadius();
	CalculateArea(x);
}

The return value of GetRadius() is stored in a variable and passed into CalculateArea(). x only exists act as the interface between the two functions.

We can do better.

Doing Better

We merge these two instructions in one line.

calculateArea( getRadius() ); // A function is the argument into another function

The return value of getRadius() is used as the argument in calculateArea(). This is possible since the function returns a value.

In this program, getRadius() is intended to be used alongside CalculateArea(). Embedding it in one line makes this relationship explicit.

NOTE: We’re not passing in a function into another function. That is called a function pointer.

This allow us to condense our programs albeit at the expense of clarity. I like it since I’m a sucker for conciseness.

The function in the argument executes first, then the outer function:

Enter the radius: 12.5 // 1st function call
The area of the circle is 490.873 // 2nd function call