typedef usage in struct definition - c++

Question

I have problem with this code,this is a header file(stack.h) from a maze program. i was studying Stack structure, and in my documents, i couldn't understand these type of structures, can anyone explain to me why we are using typedef and how the 12th and 21st line works??
#ifndef STACK_H
#define STACK_H
#define STACKSIZE 50
typedef struct d {
int x;
int y;
int right; int left;
int down;
int up;
int camefrom;
} StackDataType, position; /// LINE 12
struct STACK{
StackDataType element[STACKSIZE]; int top;
void create();
void close();
bool push(StackDataType); StackDataType pop();
bool isempty();
};
typedef struct STACK Stack; /// LINE 21
#endif

Answer 1

I think you do not need to typedef a struct again in C++, it again defines a struct, which is unnecessary. You can just define:
struct d{
};

Answer 2

In my (considerable) experience, this almost always denotes a C programmer who has fumbled their way into C++. If these are notes from your classes, it doesn't bode well.
In the earliest "C", if you declared a struct
struct StructName {
int a;
int b;
};
This didn't declare a type name, it only declared a struct name, so to make an instance of StructName you would have to write:
struct StructName myStruct;
If you wanted to be able to omit the "StructName" part you would need to use a typedef:
struct StructName { int a, b; };
typedef struct StructName StructName;
Or you could combine these into one, somewhat confusing, statement:
typedef struct StructName { int a, b; } StructName;
I say confusing because if the struct definition is many lines long, it could be confused for a second C syntax which lets you declare an instance of a Struct after defining the type:
struct StructName { int a, b; } StructName;
// aka
struct StructName { int a, b; };
struct StructName StructName; // local variable, StructName of type struct StructName
// declare a VARIABLE called StructName which is of type anonymous-struct.
struct { int a, b; } StructName;
One problem with this is that you can't use the typedef'd name in the structure declaration:
// Won't compile because 'List' isn't declared until the end.
typedef struct list_structure { List* next; int a; } List;
// Won't compile because you have to remember to say 'struct List'
typedef struct List { List* next; int a; } List;
// Compiles
typedef struct list_structure { struct list_structure* next; int a; } List;
This confused a lot of C programmers. Enough so that many C programmers will tell you that the definition of a struct is
typedef struct tag_name { /* struct details */ } structname;
//e.g.
typedef struct tagStructName { int a, b; } StructName;
C++ inherited all of this, but also went ahead and made the typedef implied for you:
// doesn't compile as C, will compile as C++
struct List {
List* next;
int a;
};
To see it not compiling as C: http://ideone.com/3r9TRy
In C++, declaring something as a class is exactly the same as declaring it a struct, with one change:
class List {
List* next;
public:
int a;
};
Is EXACTLY as though you had written:
struct List {
private:
List* next;
public:
int a;
};
There's no other difference between a struct and a class in C++.

Answer 3

I can see two problems: The first is the mysterious symbol  in the definition of the d structure. The second is that you use typedef for that structure too, but have something after the typename StackDataType. The second error you get is probably just because of the first one, it's very common in C and C++ to get errors in unrelated lines because of previous errors.
Besides, in C++ you don't really need typedef for structures, as they are the same as classes so doing e.g. struct StackDataType {...}; will allow you to use StackDataType as a type.

Answer 4

What's going on is that essentially the typedef is being used to create a shorthand way to refer to the given structure.
So in this example, both StackDataType and position are shorthand references to what is formally declared as struct d, and Stack is a shorthand reference to what is formally declared as struct STACK.
Generally speaking, this allows for cleaner code referencing these structures. E.g., instead of having to write:
struct STACK var;
to declare an instance of this structure, you can just use:
Stack var;
You can declare a typedef either at the same point at which you declare the type (as in the first example), or you can declare it later (as in the second).