Declaring variables inside a struct in c [duplicate] - c

Question

This question already has answers here:
Why can't we initialize members inside a structure?
(6 answers)
Closed 7 years ago.
I wanted to make an object oriented preprocessor to my programming language which converts my language into C (like early C++). And I want to simulate the classes with structures. And the question is: how can I declare a variable inside a struct like this:
typedef struct { //equivalent of class
int a = 5;
int (*sub)(int) = &int_sub; //function of a class, uses an external declared function
} class_name;
I tried the code above but the compiler wrote this:
error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘=’ token
void (*sub)(int) = &int_sub;
I have two questions:
Can I declare a variable inside a struct?
If yes, how?

Answer 1

You can't assign a pointer value inside a struct definition. You could use a function to init it.
typedef struct { //equivalent of class
int a;
int (*sub)(int);
} class_name;
int int_sub (int a)
{
// your stuff
return 0;
}
int main()
{
class_name myClassVariable;
myClassVariable.a = 5;
myClassVariable.sub = int_sub;
printf("class_name.a = %d\n", myClassVariable.a );
printf("class_name.sub = %p\n", myClassVariable.sub );
printf("int_sub address = %p\n", int_sub );
return 0;
}
Or, as shown in artm answer, you could init your allocated variable:
class_name my_struct = { .a = 5, .sub = int_sub };

Answer 2

Alternatively, you can also initialize the variable of your struct type.
int func( int a ){}
typedef struct {
int a;
int (*sub)(int);
} class_name;
class_name my_struct = { .a = 5, .sub = func };

Answer 3

I take it your question is not about how to declare but how to initialize a structure member.
Define the class as opaque type in the h file. Use typedef for function pointers.
h file
// opaque type declaration
typedef struct class_name class_name;
// types used by this class
typedef int sub_func_t (int);
// member functions of the class
class_name* class_init (int a, sub_func_t* sub);
Then initialize it from inside its constructor:
c file
struct class_name { //equivalent of class
int a;
sub_func_t* sub;
};
class_name* class_init (int a, sub_func_t* sub)
{
class_name* new_class = malloc(sizeof(*new_class));
assert(new_class != NULL);
*new_class = (class_name){a, sub};
return new_class;
}