Getting the sizeof DATA TYPES without using "sizeof" operator

Hi Folks,

Is there any way out to know the sizeof any DataTYPE without using "sizeof" operator.

My requirement is very simple.

Code:

---


#include <stdio.h>

int ReturnSize( // the Data Type argument to be passed, may be CHAR, INT, FLOAT, DOUBLE or even STRUCTURE)
{
      // Calculates the sizeof the argument WITHOUT using "SIZEOF" operator and returns the size

}


int main()
{ 
      int size;
      size = ReturnSize( // Can be CHAR/INT/FLOAT/DOUBLE/STRUCTURE);
}

---

ReturnSize would need to be a macro if it can accept any data type as an argument. I suppose this can be done by seeing how much a pointer moves when it is incremented.

What is this for?

In short: No!

What is the problem you are actually trying to solve?

The language C (and C++) doesn't have a way to describe the size of an arbitrary type without help from the compiler, which is what the sizeof() "function" gives. There is no way, at runtime, to calculate the size of an object.

Further, as King Mir pointed out, you can not pass arbitrary types to a function in C.

You could of course do something like this, if you don't want to use sizeof:

Code:

---

#define SizeOf(typex) size_t vval__; typex *x__; x__ = malloc(10000), vval__ = (char *)&x__[0] - (char *)&x__[1], free(x__), vval__

---

But it's pig ugly, won't necessarily work everywhere [among other things it relies in being wihtin braces in if-statements, and there's no (easy) way to change that].

Does this solve your problem? Probably not.

An explanation of the bigger picture would probably be more helpful.

--
Mats

hi mats, I really appreciate ur comments.
BUT< let me know if this is gonna work:)...

Code:

---


struct point {
    long x, y;
};

int main()
{
    struct point pt = {0}, *ppt = &pt;
    unsigned char *p1 = NULL, *p2 = NULL;
    size_t size = 0;

    p1 = (unsigned char*)(ppt);
    p2 = (unsigned char*)(++ppt);
    size = p2 - p1; // size is now 8 bytes (2 longs)
    // same as sizeof(struct point) or sizeof(pt)

    return 0;
}

---

Yes, I'm a bit stupid this early in the morning. You could of course use a 0-value for the pointer, rather than malloc some memory.

I still don't see how this is "better" than sizeof.

--
Mats

You can by yourself see the overhead of how to calculate the size of an datatype. You need more than one variable of the same datatypes to find the size, when sizeof does it for you. So what was the requirement of this in your application.

ssharish

>let me know if this is gonna work
No. You can't portably do this in C because of the padding rules for data types (and structures in the case of your example). Tell your teacher that sizeof is there to provide a portable solution to a problem that is otherwise not possible. There's no reason not to use it, and as an assignment this doesn't teach you anything except silly tricks that are subtly broken.