I have defined a structure to hold a group of values of any type, that's why the void **data. I would really appreciate if someone helps me in how to print the values pointer by an array of void pointers.
Here is the structure.
typedef struct item{
void **data;//array of void pointer to store names of the group-members
int count;//number of members in the group
struct item *next; //pointer to the next group in the pool queue
} group;
Now, let say a group is created as follows:
group *gp = (group*) malloc(sizeof(group));
void *a[2];
a[0] = (void*) "John";
a[1] = (void*) "Jim";
gp->data = a;
gp->count = 2;
At some point, probably inside another function, if I need to print the array of values of a group node, what code should I write? Here is such a function:
void print_group_node(group *gp)
{
//I have the check for vp==NULL here
void **vp = gp->data;
int c = gp->count;
int i;
for(i = 0; i < c; i++)
printf();//what should I write here
//other codes here
}
Thank you.
If the data could be any type of variable. You probably need one more field similar to data_type
below in the structure group
to tell what type data
has.
typedef struct item{
....
int data_type; // 0: int, 1: char * (string), ...
} group;
And use appropriate printf()
as per it.
void print_group_node(group *gp)
{
//I have the check for vp==NULL here
void **vp = gp->data;
int c = gp->count;
int i;
for(i = 0; i < c; i++)
switch (gp->data_type) {
case 0:
printf("%d \n", (int*)vp[i])
break;
case 1:
printf("%s \n", (char *)vp[i])
break;
Default:
printf("Unknown type");
break;
}
}
As currently your data points to strings printf()
needs "%s"
to know that it needs to print string. And then typecast items in vp[] to get rid of the warning given by printf()
that you are passing incompatible pointer.
printf("%s \n", (char *)vp[i])
You cannot print values of arbitrary type in a general fashion. You could somehow store the type information and customize your print
function, so that it supports all the possible types.
I suppose that this code snippet is in function main.
group *gp = (group*) malloc(sizeof(group));
void *a[2];
a[0] = (void*) "John";
a[1] = (void*) "Jim";
gp->data = a;
gp->count = 2;
and that you will access data members of the dynamically allocated structure while local array void *a[2];
is alive. Otherwise it would be better to allocate additional memory that would be pointed to by gp->data. For example
group *gp = (group*) malloc(sizeof(group));
gp->data = malloc( 2 * sizeof( void * ) );
gp->data[0] = "John";
gp->data[1] = "Jim";
gp->count = 2;
In this case that to output these data in some other function you need to know the actual type of pointer gp->data that to use an appropriate casting.
Thus if you know exactly that the actual type of data is char **
then in some other function you can write
for(i = 0; i < c; i++)
printf( "%s\n", ( char * ) gp->data[i] );
Otherwise you need to enumerate all types that you are going to use with your structure and to add a corresponding data member that will be used in determination of the actual type of the pointer.
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