Hiding members in a C struct

Question

I've been reading about OOP in C but I never liked how you can't have private data members like you can in C++. But then it came to my mind that you could create 2 structures. One is defined in the header file and the other is defined in the source file.

// ========================================= // in somestruct.h typedef struct {   int _public_member; } SomeStruct;  // ========================================= // in somestruct.c  #include "somestruct.h"  typedef struct {   int _public_member;   int _private_member; } SomeStructSource;  SomeStruct *SomeStruct_Create() {   SomeStructSource *p = (SomeStructSource *)malloc(sizeof(SomeStructSource));   p->_private_member = 42;   return (SomeStruct *)p; } 

From here you can just cast one structure to the other. Is this considered bad practice? Or is it done often?

Answer 1

sizeof(SomeStruct) != sizeof(SomeStructSource). This will cause someone to find you and murder you someday.

Answer 2

Personally, I'd more like this:

typedef struct {   int _public_member;   /*I know you wont listen, but don't ever touch this member.*/   int _private_member; } SomeStructSource; 

It's C after all, if people want to screw up, they should be allowed to - no need to hide stuff, except:

If what you need is to keep the ABI/API compatible, there's 2 approaches that's more common from what I've seen.

• Don't give your clients access to the struct, give them an opaque handle (a void* with a pretty name), provide init/destroy and accessor functions for everything. This makes sure you can change the structure without even recompiling the clients if you're writing a library.

• provide an opaque handle as part of your struct, which you can allocate however you like. This approach is even used in C++ to provide ABI compatibility.

e.g

 struct SomeStruct {   int member;   void* internals; //allocate this to your private struct  };