In the case of call-by-reference, a pointer reference to a variable is passed into
a function instead of the actual value. The function's operations will effect the
variable in a global as well as local sense. Call-by-value (C's method of parameter
passing), by contrast, passes a copy of the variable's value into the function. Any
changes to the variable made by function have only a local effect and do not alter
the state of the variable passed into the function. 


Does C really have pass by reference?

No.

C uses pass by value, but it can pretend doing pass by reference, by having functions
that have pointer arguments and by using the & operator when calling the function.
This way, the compiler will simulate this feature (like when you pass an array to
a function, it actually passes a pointer instead). C does not have something like
the formal pass by reference or C++ reference parameters. 


How will you decide whether to use pass by reference, by pointer and by value?

The selection of the argument passing depends on the situation.


    * A function uses passed data without modifying it:


 o If the data object is small, such as a built-in data type or a small structure
then pass it by value. 
 o If the data object is an array, use a pointer because that is the only choice.
Make the pointer a pointer to const. 
 o If the data object is a good-sized structure, use a const pointer or a const reference
to increase program efficiency. You save the time and space needed to copy a structure
or a class design, make the pointer or reference const. 
 o If the data object is a class object, use a const reference. The semantics of
class design often require using a reference. The standard way to pass class object
arguments is by reference. 


    * A function modifies data in the calling function:


 o If the data object is a built-in data type, use a pointer. If you spot a code
like fixit(&x), where x is an int, its clear that this function intends to modify
x. 
          o If the data object is an array, use the only choice, a pointer.
          o If the data object is a structure, use a reference or a pointer.
          o If the data object is a class object, use a reference.