pointer to function

I need to create a ADT that is non data type specific. To print I was going to point to a function provided by the user that would be specific to his data type. This is for an assingment and I will soon become the "user". Can someone tell me how to define everything so it will jive? I don't understand how to send the pointer to my ADT or how to define the ADT function in the prototype.

Here are some bits of information

Code:

---

#include <stdio.h>

// forward declaration for structure
struct my_struct;

// a pointer to a print function, taking a ptr to an instance
typedef void (*printfn)( struct my_struct *instance );

// a structure, containing a ptr to a function to print it
typedef struct my_struct {
    int    a;
    char    b;      // member variables
    printfn print;  // member functions
} my_struct;

void my_print_function ( my_struct *inst ) {
    printf( "%d %c\n", inst->a, inst->b );
}

int main ( ) {
    my_struct fred = {
        1, '2', my_print_function
    };
    fred.print( &fred );
    return 0;
}

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Though it looks to me like C++ would be better for you.