Why is `typedef void * COMPLEX` used in this interface?

Question

I have a program and I can't understant how it works. Here is a part of it. I don't understand the line typedef void *COMPLEX, the command this and why the struct COMPLEX_IMPL is being used.

#ifndef _COMPLEX_H 
#define _COMPLEX_H 

typedef void *COMPLEX; 

COMPLEX NewCOMPLEX (double a, double b ); 

void DeleteCOMPLEX(COMPLEX this ); 

double GetA (COMPLEX this ); 

double GetB (COMPLEX this ); 

COMPLEX AddComplex (COMPLEX c1, COMPLEX c2, COMPLEX res); 
COMPLEX MultComplex (COMPLEX c1, COMPLEX c2, COMPLEX res); 

#endif /* _COMPLEX_H */

#ifndef _COMPLEX_H 
#define _COMPLEX_H 

typedef void *COMPLEX; 

COMPLEX NewCOMPLEX (double a, double b ); 

void DeleteCOMPLEX(COMPLEX this ); 

double GetA (COMPLEX this ); 

double GetB (COMPLEX this ); 

COMPLEX AddComplex (COMPLEX c1, COMPLEX c2, COMPLEX res); 
COMPLEX MultComplex (COMPLEX c1, COMPLEX c2, COMPLEX res); 

#endif /* _COMPLEX_H */

---

#include <stdio.h> 
#include "complex.h" 

struct COMPLEX_IMPL { double a; double b; }; 

double GetA(COMPLEX this) { 
    struct COMPLEX_IMPL *this_impl = (struct COMPLEX_IMPL*)this; 
    return this_impl->a; 
}

Answer 1

typedef defines a name for a type. So

typedef void *COMPLEX;
COMPLEX z;

is equivalent to

void *z;

A pointer type normally indicates what kind of data the pointer points to. void * is an exception: it's a way to have a pointer without saying what the type of the value it points to is. You can freely assign any kind of pointer to a void * pointer and back.

void * pointers are normally used in generic library functions that must work with data of any type. For example, consider the standard library function memcpy:

void *memcpy(void *dest, const void *src, size_t n);

You pass that function a pointer to an object of any type src, a pointer to another object (which is usually, but not always, of the same type) dest, and a number of bytes to copy. The function copies the bytes, it doesn't care what the bytes mean, so it's enough to pass two pointers-to-an-unspecified-type.

The use of void * here is not good or common programming practice. A complex number is represented as its real part and its imaginary part:

struct COMPLEX_IMPL { double a; double b; };

A typical complex number library would make this the COMPLEX type.

The code you posted hides the implementation of the COMPLEX type. The fact that complex numbers are implemented as a structure containing two double members is only apparent in complex.c. Users of the library only see that a COMPLEX is a pointer to something. This is a form of data abstraction: hiding the representation details of a data type. But it's poorly done: with this definition, any pointer to anything can be assigned to a COMPLEX. The normal way is to use an incomplete structure, which is declared and visibly a structure but whose members are not specified. In complex.h, you would write:

struct COMPLEX_IMPL;
typedef struct COMPLEX_IMPL *COMPLEX;

That way, the only way to legally create a COMPLEX_IMPL is through the functions provided by complex.h, but a variable of type COMPLEX is visibly a pointer to a representation of a complexe number as defined in complex.c.

Oh, and this is an ordinary variable name.

Answer 2

typedef void *COMPLEX;

makes COMPLEX an alias for the type void *. (This is evil, btw., since you shouldn't use a pointer typedef and a void one effectively turns off type checking.)

There is no "command" this, that's just the name of an argument.

The struct is used to keep the a and b members together. That's what structs are for.

Answer 3

• typedef void* COMPLEX makes 'COMPLEX' an alias for the void pointer type 'void*'
• COMPLEX_IMPL is use because a complex number has two parts, so this groups them together
• 'this' is a variable

I think a tutorial in C would benefit you greatly here.

Answer 4

typedef void*

use to define function in dll which used by your project