The Function Body

The body of the function is a block consisting of the statements that implement the function, along with the declarations of any local variables. For functions that return a value to their caller, at least one of those statements must be a return statement: 

return expression;

Executing the return statement causes the function to immediately to return to its caller, passing back the value of the expression as the value of the function.

Functions that return a value to their caller are called fruitful functions, because they can be treated as an operand in expressions. Functions can return values of any type. Once you have defined a fruitful function, it can be used as if it were a value. For instance, the f2c program calls convert() like this: 

double celsius = convert(temperature);

In this case temperature is the argument that is used to initialize the parameter temp.

Functions do not need to return a value. Such a function is often called a procedure. Procedures must have some kind of side-effect, such as printing, to be useful.

To define a procedure, use void as the function's return type. Procedures ordinarily finish by reaching the end of the statements in the body, but you may leave the procedure early by executing a return statement by itself.

Two C++ Function Pitfalls

  • Unlike Java and C#, unreachable code is not illegal. (It is a bug, though!)
  • If you forget to add a return statement to a fruitful function, your code will still compile. The actual returned value will be random. This may cause your program to crash, or simply act erratically.