Thread: sizeof(struct.member) problem

You might want to look at this code and the output:

Code:

#include <iostream>
using namespace std;

int main()
{
	int test;

	cout<<sizeof(int)<<endl;
	cout<<sizeof(test)<<endl;
	
	const int CONST_SIZE=20;
	char name1[CONST_SIZE];

	cout<<CONST_SIZE<<endl;
	cout<<sizeof(name1)/sizeof(name1[0])<<endl;

	return 0;
}}

That wasn't his question. He asked for the size of a structmember without having to create an instance of the struct, if I got it right

I guess I would try something like sizeof(TMyStruct::iMyIntMember);

edit: whoop, doesn't work unless you want a to be static...

uh....

how bout sizeof(int) and sizeof(char)*CONST_SIZE ?

Or, use a pointer:

Code:

int main(void)
{
  mystruct *p;
  cout <<sizeof(p->item1) <<endl;
  return(0);
}

>How do I find the length of a member of mystruct without first declaring it etc?

I normally don't do C++, but I did encounter a similar situation recently in C. And since I haven't seen it mentioned yet and I'm curious if there might be any problems with this in C++, here we go:

Code:

/* I'll stick with C and let you C++ folks C++-ize it properly. */
#include <stdio.h>

#define CONST_SIZE 15

struct mystruct
{
    int  item1;
    int  item2;
    char name1[CONST_SIZE];
    int  item3[CONST_SIZE];
};

int main(void)
{
    printf("sizeof(item1) = %lu\n", (long unsigned)sizeof(((struct mystruct*)0)->item1));
    printf("sizeof(item2) = %lu\n", (long unsigned)sizeof(((struct mystruct*)0)->item2));
    printf("sizeof(name1) = %lu\n", (long unsigned)sizeof(((struct mystruct*)0)->name1));
    printf("sizeof(item3) = %lu\n", (long unsigned)sizeof(((struct mystruct*)0)->item3));
    return 0;
}

/* my output
sizeof(item1) = 4
sizeof(item2) = 4
sizeof(name1) = 15
sizeof(item3) = 60
*/

There seems to be some support for this method here and here.

And before I'm told that this dereferences a NULL pointer, I believe a C reply I'd tracked down (somewhere) stated that the sizeof operator does not evaluate its argument (except for variable-length arrays in C99), so no dereferencing is actually done. Whether this is the case for Standard C++ I don't know (I only briefly skimmed a draft).

[edit]
Fixed a minor typo and highlighted the construct.
[/edit]

that's pretty slick actually. I never had a need to do that though. I usually will just sizeof(The Type) instead.

I still don't see it. Do you have a "keyfield" of different types in different records? Suppose it is int.

CompareByKeyfield(record1.keyfield, record2.keyfield);

You definitely don't need to know the size here. Additionally, if you used classes (this is the C++ forum after all) you could do a compare like this:

if(record1 > record2)

where the overloaded operator needs to know the sizeof

Code:

int operator > (Record & record)
{
sizeof(keyfield) //do something with the size

then you clearly don't need any of what we've been talking about anyway!