Styjun

I'm trying to recreate an encapsulation principle in ANSI-C for educational purposes. What I essentially did was making some structure in .c file:

struct _private

 unsigned char SizeInBytes;
 unsigned char* matrix;
 struct Stack* S;
 unsigned char ByteX;
;

which represented variables I wanted to be unseen. Then in .h file inside the struct (class) I created an opaque pointer:

struct Maze

 void* _private;
;

which I assign later in constructor function like this:

void* Maze_ctor(void* self, va_list *ap)

 struct Maze* this = self;

 this->DimX = va_arg(*ap, unsigned char);
 this->DimY = va_arg(*ap, unsigned char);

 this->_private = &(struct _private) // passing address of struct to void*
 
 .SizeInBytes = this->DimX*this->DimY >> 1,
 .S = new(Stack),
 .ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8
 ;
 // 
 private.matrix = (unsigned char*)malloc(private.ByteX*this->DimY);
 S = new(Stack); // this in my new() and it works similar to C++ new

 for (int i = 0; i < private.ByteX*this->DimY; i++)
 *(private.matrix + i) = 0;

At this point everything works fine, but then I'm trying to call the Next() method:

int Next(void* self, ...)

 struct Maze* this = self;

 struct _private *r = this->_private;

 short t;

 toBinary(this); // after this point the struct private breaks

the prototype of toBinary() is:

void toBinary(const void* self)

 // somehow char local is defined and equals to 204??
 struct Maze *this = self;
 struct _private *r = this->_private;

 unsigned char local; // right after this point SizeInBytes equals to 204!
...

the question is: how to fix this problem. Using C++ is prohibited!
for the interested ones: here is new()

void* new(const void* _class,...)

 const struct Class* class = _class; // we need to convert pointer from void* to class* safely
 void *p = calloc(1, class->size); // allocation of memory for class .using size param

 assert(p); // if Null -> throw an error
 *(const struct Class**)p = class; // safe assignment of class pointer to (value) of p, to have memory and built in funcs
 if (class->ctor) // if has constructor with some dynal in it, execute with varargs on its input
 
 va_list ap;
 va_start(ap, _class); // 
 p = class->ctor(p, &ap); // pass arguments as a list of pointers.
 va_end(ap);
 
 return p; //returns a pointer to class pointer (weird but worx)

As pointed out in the comment, the problem is that you created a local object and assign it to a pointer this. Outside that function, the value of this is not valid.

You code,

void* Maze_ctor(void* self, va_list *ap)

//....
// this creates a temporary object and will be destroyed after Maz_ctor returns.

this->_private = &(struct _private) // passing address of struct to void*
 
 .SizeInBytes = this->DimX*this->DimY >> 1,
 .S = new(Stack),
 .ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8
;
// ---

Thanks to @Sami Kuhmonen for pointing out on dynamic allocation and @CS Pei for mistake analysis. The thing I did to fix this is:

struct Maze

 char _private[32]; // allocate the memory size of struct(32)



// assign values to void ptr
private.SizeInBytes = this->DimX*this->DimY >> 1;
private.S = new(Stack);
private.ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8;
private.matrix = (unsigned char*)malloc(private.ByteX*this->DimY);

this now works as intended, but a little slower