accessing struct element from another struct

**creek23** · 06-23-2010

Hello, I'm not new to programming. I've been developing games with BASIC (FreeBASIC), and Flash games with ActionScript. I even developed a scripting language for beginning programmers to make games.

Now, I decided to learn C to be able to make games for handheld consoles.

I have this code below, where I keep having error message saying:
error: request for member 'value' is something not a structure or union

What I wanted to do is be able to have an array of resizeable strings. I'm aware that there's no String datatype in C. It is why I have char *value in struct Foo. It will serve as my resizeable string in my program.

Now, struct Bar will contain a resizeable array of struct Foo, thus, Foo **foo.

PHP Code:


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

typedef struct {
    int etc1;
    char *value;
} Foo;

typedef struct {
    char *etc2;
    Foo **foo;
} Bar;

int main() {
    Bar bar;
    
    bar.foo = (Foo **) malloc(sizeof(Foo) * 2);
        bar.foo[0] = (Foo *) malloc(sizeof(Foo));
        bar.foo[0].value = (char *) malloc(sizeof("howdy"));
            bar.foo[0].value = "howdy";
        
        bar.foo[1] = (Foo *) malloc(sizeof(Foo));
        bar.foo[1].value = (char *) malloc(sizeof("hello world"));
            bar.foo[1].value = "hello world";
    
                printf("bar.foo.value[0] = %s\n", bar.foo[0].value);
                printf("bar.foo.value[1] = %s\n", bar.foo[1].value);
    
        free(bar.foo[0].value);
        free(bar.foo[0]);
        
        free(bar.foo[1].value);
        free(bar.foo[1]);
    
    free(bar.foo);
    
    return 0;
}

In ActionScript, accessing value inside foo would be foo.value. And it follows that accessing foo inside bar would be bar.foo. So I was hoping, I could access value via bar through bar.foo.value. And given that foo is a resizeable array, I was hoping this would work: bar.foo[0].value.

What am I doing wrong? Is this even possible? Or should I declarare a separate Foo object and pass the pointer to it and assign the value?

**Bayint Naung** · 06-23-2010

>Now, struct Bar will contain a resizeable array of struct Foo, thus, Foo **foo.
Shouldn't it be Foo *foo;
You've an error message but which line???

Code:

		bar.foo[0].value = (char *) malloc(sizeof("howdy"));
		bar.foo[0].value = "hello world";

This is not really what you want. You are effectively leaking memory.
I'd suggest you read a good tutorial on C.

**creek23** · 06-23-2010

oops.

That was just an error in my attempt to simplify what I'm currently working on. It should have been

PHP Code:


        bar.foo[0].value = (char *) malloc(sizeof("howdy"));
            bar.foo[0].value = "howdy";
        
        bar.foo[1] = (Foo *) malloc(sizeof(Foo));
        bar.foo[1].value = (char *) malloc(sizeof("hello world"));
            bar.foo[1].value = "hello world";

Anyway, the error is on all lines trying to access .value.

**Kennedy** · 06-23-2010

strcpy is what you need. You cannot assign a string to a location like that, however, you could assign a string literal to a pointer -- but I don't think that's what you want to do.

**creek23** · 06-23-2010

whoopie! got it! changed .value to ->value.

PHP Code:


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

typedef struct {
    int etc1;
    char *value;
} Foo;

typedef struct {
    char *etc2;
    Foo **foo;
} Bar;

int main() {
    Bar bar;
    
    bar.foo = (Foo **) malloc(sizeof(Foo) * 2);
        bar.foo[0] = (Foo *) malloc(sizeof(Foo));
        bar.foo[0]->value = (char *) malloc(sizeof("howdy"));
            bar.foo[0]->value = "howdy";
        
        bar.foo[1] = (Foo *) malloc(sizeof(Foo));
        bar.foo[1]->value = (char *) malloc(sizeof("hello world"));
            bar.foo[1]->value = "hello world";
    
                printf("bar.foo.value[0] = %s\n", bar.foo[0]->value);
                printf("bar.foo.value[1] = %s\n", bar.foo[1]->value);
    
        free(bar.foo[0]->value);
        free(bar.foo[0]);
        
        free(bar.foo[1]->value);
        free(bar.foo[1]);
    
    free(bar.foo);
    
    return 0;
}

**creek23** · 06-23-2010

Originally Posted by Kennedy

strcpy is what you need. You cannot assign a string to a location like that, however, you could assign a string literal to a pointer -- but I don't think that's what you want to do.

well, you could. as long as the char pointer has memory allocated of just about the same size of the string value being assigned.

in it's simplest form:

PHP Code:


[code]

#include <stdio.h>



int main() {

    char *value = (char *) malloc(sizeof("howdy"));

    value = "howdy";

    printf("%s", value);

}

[/code]

should work. (and it does)

**creek23** · 06-23-2010

Originally Posted by Bayint Naung

>Now, struct Bar will contain a resizeable array of struct Foo, thus, Foo **foo.
Shouldn't it be Foo *foo;

hmm... let me take a look at it again. thanks for the tip.

**creek23** · 06-23-2010

Thanks Bayint Naung. I edited the code and it does work with only Foo *foo and not Foo **foo, thus no need to malloc both array instance.

PHP Code:


[code]
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int etc1;
    char *value;
} Foo;

typedef struct {
    char *etc2;
    Foo *foo;
} Bar;

int main() {
    Bar bar;
    
    bar.foo = (Foo *) malloc(sizeof(Foo) * 2);
        bar.foo[0].value = (char *) malloc(sizeof("howdy"));
            bar.foo[0].value = "howdy";
        
        bar.foo[1].value = (char *) malloc(sizeof("hello world"));
            bar.foo[1].value = "hello world";
    
                printf("bar.foo[0].value = %s\n", bar.foo[0].value);
                printf("bar.foo[1].value = %s\n", bar.foo[1].value);
    
        free(bar.foo[0].value);
        
        free(bar.foo[1].value);
    
    free(bar.foo);
    
    return 0;
}[/code]

**creek23** · 06-23-2010

Thanks Bayint Naung. I edited the code and it does work with only Foo *foo and not Foo **foo, thus no need to malloc both array instance.

PHP Code:


[code]
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int etc1;
    char *value;
} Foo;

typedef struct {
    char *etc2;
    Foo *foo;
} Bar;

int main() {
    Bar bar;
    
    bar.foo = (Foo *) malloc(sizeof(Foo) * 2);
        bar.foo[0].value = (char *) malloc(sizeof("howdy"));
            bar.foo[0].value = "howdy";
        
        bar.foo[1].value = (char *) malloc(sizeof("hello world"));
            bar.foo[1].value = "hello world";
    
                printf("bar.foo[0].value = %s\n", bar.foo[0].value);
                printf("bar.foo[1].value = %s\n", bar.foo[1].value);
    
        free(bar.foo[0].value);
        free(bar.foo[1].value);
    
    free(bar.foo);
    
    return 0;
}
[/code]

**Kennedy** · 06-24-2010

Look, you have a memory leak. You are assigning a string literal over a malloced pointer. You are loosing that memory. If you assign a string literal to a pointer, it gets the ADDRESS of that memory, not a strcpy of that memory.

edit:

Code:

root@fw03:~# vim test.c
root@fw03:~# gcc -os test.c
root@fw03:~# ./s
st = 80484e0, tmp = 804a008
root@fw03:~# cat test.c
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
        char *st, *tmp;

        st = malloc(100);

        tmp = st;
        st = "Hello World";

        printf("st = %x, tmp = %x\n", st, tmp);
        free(tmp);
        return 0;
}

**creek23** · 06-24-2010

cool!

thanks for correcting me. and for a snippet on testing memory leaks.

thanks.

Thread: accessing struct element from another struct

Thread Tools

Search Thread

Display

accessing struct element from another struct

Similar Threads

Problem accessing the struct object.

Link List math

Looking for a way to store listbox data

Struct with a ptr to a dynamically allocated array of other structures :(

Address of Structure element, and struct malloc

Tags for this Thread