Thread: type swapping?

Okay, I'm writing a function where it's going to have to deal with three possible data types in an identical manner...

currently, I'll have to do the following

Code:

   void *arrayPtr;
   if(datatype == 1)
   {
        arrayPtr = calloc(5, sizeof(int));
        int holder;
    }
    else if(datatype == 2)
    {
        arrayPtr = calloc(5, sizeof(double));
        double holder;
    }
    else if(datatype == 3)
    {
        arrayPtr = calloc(5, sizeof(float));
        float holder;
    }

now, is it possible to create a function or macro that can let me replace the top with something like this?

Code:

void *arrayPtr;
arrayPtr = calloc(5, sizeof(TYPECONV(datatype)));
TYPECONV(datatype) holder;

Originally Posted by Sebastiani

Code:

#define TYPECONV(t)\
(t==1?sizeof(int):\
 t==2?sizeof(double):\
 t==3?sizeof(float):\
 sizeof(char))

This works for the sizeof part, but the tricky part is this:

Code:

TYPECONV(t) newvariable //<---- Declaring a new variable in the scope with the same datatype...

Is this possible at all in C, or should I stop barking up the wrong tree?

Other than the part where I can't find enough information to determine if this works, unions sounds just like what I need...

Code:

#define TYPESWAP(x)\
(t==1?swapvar.intvalue\
 t==2?swapvar.floatvalue:\
 swapvar.doublevalue)


union vartype
{

int intvalue;
float floatvalue;
double doublevalue;
}swapvar;

will sizeof(swapvar.intvalue) give me the size of an int variable?
what about union vartype *swapPtr?
is can *swapPtr be used as a pointer to a variable with the datatype of one of the members?

Nevermind, tried it out myself, and the only thing I ever got was errors...
When I compile, I get this error:

20 C:\Dev-Cpp\prototyping\typeconv.c invalid lvalue in unary `&'

Posting code...
Any ideas how to make this work?

Code:

#define TYPESWAP(t)\
(t==1?swapvar.intvalue:\
 t==2?swapvar.floatvalue:\
 swapvar.doublevalue)


union vartype
{

int intvalue;
float floatvalue;
double doublevalue;
}swapvar;

int main(int argc, char *argv[])
{

printf("inserting test int: ");
scanf("%d", &TYPESWAP(1));
printf("%d\n", TYPESWAP(1));
system("PAUSE");	
return 0;
}

Which actually defeats the reason I wanted to try that out in the firstplace, so nevermind.
Thanks anyway...

Don't ever say nevermind there is always a way

Code:

typedef union vartype{
int intvalue;
float floatvalue;
double doublevalue;
} vartype;

now

Code:

var=vartype{1}; /*this is an int*/

I think this would work. But really the union is sizeof largest element. Will this work(no compiler on me!)

>Which actually defeats the reason I wanted to try that out in the firstplace, so nevermind.
Using scanf and printf in a type independent way is virtually impossible. Even where you don't need a cast, you still need to match the format modifier to the desired type.

actually that was a program to test and see if I can make the program automatically treat the union as one of the members without having to specifically calling the member.

essentially, what I was looking for, was either a way to make the union call the apropriate member by doing something like this:

myownfunction( SELFCONVERTINGUNION(datatype));

as opposed to having to do this

Code:

switch(datatype)
{

case 1:
 
myownfunction(unionvar.member1);
break;
case 2:

myownfunction(unionvar.member2);
break;
.
.
.
}