In some cases, you may need to use a type before you have actually defined it. As long as you do not use the members, you can declare a type before defining it:
struct time_of_day; void print_day(time_of_day time);
This could be declared in a header, where it says that there is a function defined somewhere else that takes a time_of_day record and prints it out. To be able to declare the print_day function, you have to have declared the time_of_day name. The time_of_day struct must be defined somewhere else in your code before the function is defined, otherwise you will get an undefined type error.
There is, however, an exception: a type can hold pointers to instances of the same type before the type is fully declared. ...
