Default constructor in C

Question

Is there a way to have some kind of default constructor (like a C++ one) for C user types defined with a structure?

I already have a macro which works like a fast initializer (like the one for pthread_mutex) but I wanted to know if you can by any chance have some (or all) fields of a struct filled at declaration.

For instance, with the pthread_mutex example, I would like

pthread_mutex_t my_mutex; 

to have the same effect as

pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER; 

Answer 1

You can create initializer functions that take a pointer to a structure. This was common practice.

Also functions that create a struct and initialize it (like a factory) - so there is never a time where the struct is "uninitialized" in the "client" code. Of course - that assumes people follow the convention and use the "constructor"/factory...

horrible pseudo code with NO error checking on malloc or free

somestruct* somestruct_factory(/* per haps some initializer agrs? */) {   malloc some stuff   fill in some stuff   return pointer to malloced stuff }   void somestruct_destructor(somestruct*) {   do cleanup stuff and also free pointer   free(somestruct); } 

Someone will probably come along and explain how some early C++ preprocessors/compilers worked to do this all in C.

Answer 2

C++ is different from C in this case in the respect that it has no "classes". However, C (as many other languages) can still be used for object oriented programming. In this case, your constructor can be a function that initializes a struct. This is the same as constructors (only a different syntax). Another difference is that you have to allocate the object using malloc() (or some variant). In C++ you would simlpy use the 'new' operator.

e.g. C++ code:

class A {   public:     A() { a = 0; }     int a; };  int main()  {   A b;   A *c = new A;   return 0; } 

equivalent C code:

struct A {   int a; };  void init_A_types(struct A* t) {    t->a = 0; }  int main() {    struct A b;    struct A *c = malloc(sizeof(struct A));    init_A_types(&b);    init_A_types(c);    return 0; } 

the function 'init_A_types' functions as a constructor would in C++.