A function that has a return value produces a value that you can assign to a variable or use in some other expression. For example, the standard C/C++ library includes a function called
sqrt() that returns the square root of a number. Suppose you want to calculate the square root of 6.25 and assign it to the variable
x. You can use the following statement in your program:
x = sqrt(6.25); // returns the value 2.5 and assigns it to x
sqrt(6.25) invokes, or calls, the
sqrt() function. The expression
sqrt(6.25) is termed a function call, the invoked function is termed the called function, and the function containing the function call is termed the calling function (see Figure 2.6).