Opaque Pointers ! - C

[Suko]

If anyone could help explain this, thanks,
If i try to compile a headerfile with the entry:-
typedef struct T *T;
I recieve this message from the compiler:-
C:\Program Files\Miracle C\Crap.h: line 1: Parse Error, expecting `'{''
'typedef struct T *T'
aborting compile
Why will this not compile? I can compile it in C++ with another compiler, but not with my C compiler, I am defining the struct in a .c file.
As far as i know this should work, anyone help?
The compiler is Miracle C Version 3.2.

[JosAH]

If anyone could help explain this, thanks,
If i try to compile a headerfile with the entry:-
typedef struct T *T;
I recieve this message from the compiler:-
C:\Program Files\Miracle C\Crap.h: line 1: Parse Error, expecting `'{''
'typedef struct T *T'
aborting compile
Why will this not compile? I can compile it in C++ with another compiler, but not with my C compiler, I am defining the struct in a .c file.
As far as i know this should work, anyone help?
The compiler is Miracle C Version 3.2.

The typedef functionality is just a quick C hack, i.e. any tentative declaration
with the word 'typedef' in front of it turns the 'variable' into an alias for the type
of the variable.

If T wasn't a struct tag name before it'll cause an error in your text. What is/was T?

kind regards,

Jos

[Suko]

The typedef functionality is just a quick C hack, i.e. any tentative declaration
with the word 'typedef' in front of it turns the 'variable' into an alias for the type
of the variable.

If T wasn't a struct tag name before it'll cause an error in your text. What is/was T?

kind regards,

Jos

Hi,

I am using it to provide a type of data hiding method,
This is my .h file :-
================================
typedef struct TMan *Man_ptr;
Man_ptr Man_Create(int w, char c, short h);
================================

This is my .c file :-
================================
#include <stdlib.h>
#include "TMan.h"

struct TMan
{
int Weight;
char Colour;
short Hieght;
};

Man_ptr Man_Create(int w, char c, short h)
{
Man_ptr result = (Man_ptr)malloc(sizeof(struct TMan));
if(result)
{
result->Weight = w;
result->Colour = c;
result->Hieght = h;
}
return result;
}
================================

Then in the Main .c file :-
================================
#include "TMan.h"

void main()
{
int i;
Man_ptr MyMan;
MyMan = Man_Create(1, 2, 3);
}
================================

There is obviously more but to simplfy it ive just shown 1 method, its trying to create an object then manipulate it with object methods, like you would in C++ classes, to hide the struct data from main, you know OOP. I just dont understand why this will not compile with my C compiler but will with C++ when this is the way to do C++ data hiding in C, or so i thought.

Any Idea's.

[weaknessforcats]

There is obviously more but to simplfy it ive just shown 1 method, its trying to create an object then manipulate it with object methods, like you would in C++ classes, to hide the struct data from main, you know OOP.

I hate to break this to you, but there are no classes in C++. There are only structs. That is, this code is identical:

struct TMan
{ 
int Weight;
char Colour;
   short Hieght;
};
class TMan
{ 
   public:
     int Weight;
     char Colour;
     short Hieght;
};
 

The underlying implementation is a struct. Now since data is to be private, this code is identical:

struct TMan
{ 
   private:
    int Weight;
    char Colour;
   short Hieght;
};
class TMan
{ 
  private:
     int Weight;
     char Colour;
     short Hieght;
};
 

And now you can write member functions:

struct TMan
{ 
   private:
    int Weight;
    char Colour;
   short Hieght;
   public:
      void SetWeight(int wt);
};
class TMan
{ 
  private:
     int Weight;
     char Colour;
     short Hieght;
  public:
      void SetWeight(int wt);
};
 

and so on.

If you specify private/public/protected for every struct member, the code is identical to a class.

Try it yourself. Just take a class that compiles and change it to a struct. It will still compile.

The only difference between a struct and a class is that the default access for a struct is public whereas the default access for a class is private.