[C] Wrapping around functions with different return types

Alien_Hominid · 02-01-2010, 02:22 AM

Hello,

I have functions which return different data like:
int, char*, double...

I also have a list of datatypes (INTEGER, TEXT, REAL...) which can be returned.

I need to map a datatype with function, which purpose is to return it. Therefore, when I determine a datatype, I would like to call the required function without doing switch all the time.

All data querying goes in a loop.

My idea is to have an array of structs like this:

Code:

struct type_func_map
{
  int type;
  void (*func_wrapper_int)(void); //void (*func_wrapper_text)(void); void (*func_wrapper_real)(void);
};

and have:

Code:

void func_wrapper_int()
{
  int data = get_int();
  printf(); //do smth
}

void func_wrapper_text()
{
  char data = get_text();
  printf(); //do smth
}

Any opinions? Is there any cleaner solution which does not pollute your code?

estabroo · 02-01-2010, 07:39 AM

Why not just make a union of your return types and have your return a variable of that union type, then you just access the member of that union that the function you just called returns

Alien_Hominid · 02-01-2010, 11:21 AM

OK, I could make this:

Code:

union my_union
{
  int a;
  char *b;
  double c;
};

and make functions store their return values into the appropriate members of this union, but I need to know which function to call when I have a datatype TEXT or datatype INTEGER given in advance. Let's say you need to call char *func1() if datatype is TEXT or int func2() if datatype is INTEGER. Is there a way to have some sort of array or structure capable holding functions with different return values.

Basically, what I want is:

Code:

my_union.a = *(funcarray[INTEGER]);
my_union.b = *(funcarray[TEXT]);

carbonfiber · 02-01-2010, 12:53 PM

For example:

Code:

#include <stdio.h>


union retval {
	int INTEGER;
	char *TEXT;
	double REAL;
};

union retval f_integer(void)
{
	return ((union retval) { .INTEGER = 42 });
}

union retval f_text(void)
{
	return ((union retval) { .TEXT = "The Answer" });
}

union retval f_real(void)
{
	return ((union retval) { .REAL = 1.21 }); // gigawatts
}


int main(void)
{
	union retval (*FT[])(void) = { f_integer, f_text, f_real };
	enum { INTEGER, TEXT, REAL };

	printf("INTEGER: %d\n", FT[INTEGER]().INTEGER);
	printf("TEXT: %s\n", FT[TEXT]().TEXT);
	printf("REAL: %lf\n", FT[REAL]().REAL);
}

I'm unsure about something. So using an array such as FT in the code above, you can "easily" decide which function to call. The problem is - what do you do with the returned value?

Alien_Hominid · 02-02-2010, 04:04 AM

So you actually made union retval returning wrappers around data processing functions.

My problem

I have data stored like this:

Code:

INTEGER | TEXT | REAL
---------------------
   1    |hello | 3.14
   3    |world | 1.27
  666   |doom  | 2.18

I can get the type of each column and there are special functions querying data from INTEGER, TEXT, REAL columns (e.get_int(), get_string(), get_real()). What I need to do is to get column type and call appropriate function to get the data. The data processing should work independently from the order of columns.

carbonfiber · 02-02-2010, 04:30 AM

Yes, and you can do that using a table such as FT from above, my question is: what do you have to do with the data once it has been retreived? Example:

Code:

union Data { int i; char *t, double r; };
enum Type { INTEGER, TEXT, REAL };
// ...

enum Type data_type = get_data_type();
union Data data = get_data(data_type);

// now what? what do you have to do with data?

I don't know if it clear where I am getting at. What I'm trying to say is that.. once you have received the data.. you still need to decide how you are going to process it. So you'll need yet more tables, or switch().

Alien_Hominid · 02-02-2010, 09:38 AM

No, there is function to display data:

Code:

void display_data(column_count, column_type, data, ...)

And your functions basically wrap my data retrieval functions, which I am hoping to avoid.

carbonfiber · 02-02-2010, 09:41 AM

Ok. Problem solved then?

Alien_Hominid · 02-02-2010, 09:46 AM

If there aren't any other way of doing the same thing without creating own wrappers, then yes.

carbonfiber · 02-02-2010, 10:00 AM

To be quite honest, I'm still trying to see the 'big picture'. Do you have different display functions for each column/data type? Do you have only one function? What prototype does it have? It is difficult to suggest design decisions without at least having a decent view of the 'global picture'.

Alien_Hominid · 02-02-2010, 10:33 AM

Ok, there is sqlite database: http://www.sqlite.org/c3ref/column_blob.html
There is a GTK function to create GtkListStore: http://library.gnome.org/devel/gtk/2...list-store-set

Now I would like to have a loop:

Code:

 for(int j = 0; j < column_count; j++)
 {
   gtk_list_store_set (store, iter, j, structure_array[j].(*func(statement, j)), -1);
 }

Func is one of these:

int sqlite3_column_int(sqlite3_stmt*, int iCol);
const unsigned char *sqlite3_column_text(sqlite3_stmt*, int iCol);
double sqlite3_column_double(sqlite3_stmt*, int iCol);

and structure consists of:

Code:

{
  type;
  function_to_call;
}