How to instantiate multiple objects C

**zahid990170** · 06-15-2011

Hi,

I have the following problem:

I have a program Tree.c that reads a text file written in a particular format. From that information, it creates a tree.

i.e. for the following text file:

Code:

It will create the following tree:

16
_____|_____
17 18
_____|_____
19 20

I have a function that returns me the pointer to the root of the tree: i.e. node 16

I can invoke this function with different text files having the configurations written in them. And, I will create various trees, and get a pointer to the roots.

Let us say,

Code:

// we have three files: 
file1, file2, file3

// and, we create three trees, 
Tree root1 = create_tree(file1);
Tree root2 = create_tree(file2);
Tree root3 = create_tree(file3);

The problem is the following:

Although the tree is create, I lose control of the root node. In the caller program, only root3 is asserted to be the root. If I remove the third call, then root2 will be asserted as the root of the tree,
i.e. the information for previous roots is overwritten.

Is there a way in C, to make this act as a class, from which we can create objects, and each one retains its properties.

Thanks,

**AndrewHunter** · 06-15-2011

How actually does your function "return" you a pointer to the root of the tree?

**MK27** · 06-15-2011

Originally Posted by zahid990170

Is there a way in C, to make this act as a class, from which we can create objects, and each one retains its properties.

Yes, altho you don't need to do that, what you have should be fine.

To be clear: you have a function, create_tree(), which returns a pointer to the root node of a tree created from a file. You call that three times, storing the pointer in 3 separate variables.

No problem -- unless there is something wrong with create_tree(). Consider:

Code:

 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *create_txt (char *txt) {
	char *rv = malloc(strlen(txt)+1);
	strcpy(rv,txt);
	return rv;
}


int main(void) {
	char *a = create_txt("one"),
		*b = create_txt("two"),
		*c = create_txt("three");

	printf("%s\n%s\n%s\n", a, b, c);

	free(a);
	free(b);
	free(c);

	return 0;
}

These pointers retain their value and so should yours.

Post the code for create_tree().

**zahid990170** · 06-15-2011

Hi,

I read in the hierarchy line by line,
The very first item in the file is the root,
root is a global variable in the Tree.c file.
The function keeps on reading lines, and inserting the nodes in the right places,
Finally, when all the file is read, branches are linked,
I get back a pointer to the root,

Btw, I have the following structure;:

Code:

#define MAX_BRANCHES 4


struct TreeNode;
typedef struct TreeNode* T_Ptr;

struct TreeNode{ 
        int val; 
	T_Ptr myBranches[MAX_BRANCHES];
	T_Ptr parent;
	int num;
};

I guess, I will have to make a separate root variable for each file I read.

Thanks,

**zahid990170** · 06-15-2011

Hi,

Thanks, your reply was helpful.

You have given a nice similar example.
What I understand is that you create a pointer from within the function "create_txt" that you finally return. So, it is different for different calls.
I guess my problem comes from the fact that "root" is a global variable. And each new call can overwrite it.

The root was made global so it becomes easy to implement different functions and refer to the root without having to pass the "root" to each of the functions.
But, I guess, I will have to do that.

**MK27** · 06-15-2011

Originally Posted by zahid990170

I guess my problem comes from the fact that "root" is a global variable. And each new call can overwrite it.

The root was made global so it becomes easy to implement different functions and refer to the root without having to pass the "root" to each of the functions.
But, I guess, I will have to do that.

Yeah, it seems a hassle, but think about it in relation to OOP syntax: if I have an object, X, with it's own internal root variable, created:

Code:

obj X = new tree(filename);

When use it, I have to do this:

Code:

X.somemethod();

Sans OO, X might be a struct containing several variables, or just one, the root pointer. When I want to use it with a global function intended to work with variables of that type:

Code:

somefunction(X);

Not much different, and in fact you will commonly find X referred to in C programming APIs as an object, and those global (or static) functions as constituting part of its class. I don't know whether this is borrowed from conventional OOP, or whether it historically preceeds it (ie, conventional OOP terms are derived from this sort of C abstraction).

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