C++ Programming - Functions - Discussion

There are two rules you must be aware of when using default arguments. First, only trailing arguments may be defaulted. That is, you can't have a default argument followed by a non-default argument. Second, once you start using default arguments in a particular function call, all the subsequent arguments in that function's argument list must be defaulted (this follows from the first rule).

During prototype declaration of a function default argument should be in right hand side in the parameter list.

Correct Answer is A, not D. Please refer page No. 132 of "The C++ Programming language" by Bjarne Stroustrup. It states that: It is possible to provide default arguments for trailing arguments only.

Arguments are defaulted from right to left but c++ expects that only the arguments on the right hand side can be defaulted.
Eg:
int f(int a, int b=10,int y=5)

when we call this function with f(25,20)
we will get a=25,b=20 and y=5

Default arguments must be at trailing side only or all arguments must be default arguments.