structures declaration and definition

[rameez ali khan]

is possible to combine structures declaration and definition in programming language

structures declaration and definition

[weaknessforcats]

Exactly what do you mean? Can to give me an example?

[donbock]

I'm not sure what your question means. If your question is...
Can a structure declaration and a structure definition (for the same structure) both be in the same source file?

The answer to that question is yes. In fact, the best way to connect instances of a structure together into a linked list is to precede the structure definition with a structure declaration:

struct s;           // structure declaration.
struct s {          // structure definition.
   struct s *next;  // declaration makes this legal.
   ...
};

[donbock]

On the other hand, if your question is...
Is there any way to combine a structure declaration and structure definition into one thing so I don't have to type both?

The answer to that question is yes. A structure definition counts as a structure declaration. You can get by using only structure definitions.

If that's true then why would you ever bother with a structure declaration? A structure declaration can be used when you want the compiler to know there is a structure, but you don't care if it knows what is in the structure or how big it is. That may not sound useful, but it works just fine if all you want is a pointer to the structure.

A structure declaration is sometimes called an opaque type declaration -- because you can't see what is inside it. A common idiom is to use an opaque type in a library API. One library function allocates a new variable, returning a pointer to it (sometimes called a handle). This pointer is then passed to the other functions in the library. The structure is defined within the library, but it is only declared in the library API. An example of this technique is the FILE* argument used in stdio.h.