What is opaque pointer?

Question

What type of conversion is not accepted in C and why?
a) char to int
b) float to char pointer
c) int to char
d) double to char

Answer 1

An opaque pointer is one in which no details are revealed of the underlying data For example, you may declare in a header file

typedef struct mystruct *p_mystruct;

which declares a type p_mystruct which is a pointer to the opaque structure struct mystruct, hence anything you declare as p_mystruct will be an opaque pointer.

Users can write code like:

p_mystruct x = NULL;

Comments

Good explanation.

Answer 2

An opaque pointer is the pointer for which programmer cannot get the internal of the variable it points (means internal of the variable being accessed is hidden). There are many ways to achieve this:

1. Use void * pointer to access the structure. Problem with this approach is that user cannot use statically allocated variable of the structure. HANDLE in Win32 is implemented in this way.
2. Declare a structure with padding of chars in the header file which will give it same size as of the original structure. In the implementation code, use the structure directly. In this approach, user will not get any hint of the internal of the structure, but can declare statically allocated variable of the structure. pthread_t is implemented in most the cases like this.