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substitute of sizeof operator

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substitute of sizeof operator

Sep 6th, 2005

I am back again with a new query...preparing for technical interview questions
---------------------------------
Without using sizeof operator,can we find sizeof datatype.

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Re: substitute of sizeof operator

Sep 6th, 2005

My first reaction would be to say that sizeof is part of the language and go down that path...

But one technique that might be used is to declare an array of a least two of the objects, declare two pointers to char; then cast & assign the address of adjactent objects to each pointer; the difference between the pointers is the size of the object in bytes.

Last edited by Dave Sinkula; Sep 6th, 2005 at 3:02 pm. Reason: Changed 'structures' to 'objects'.

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Re: substitute of sizeof operator

Sep 6th, 2005

actually awesome.....i tried it and it worked fine...thanx alot

here's the code for others
--------------------------

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include<iostream>
using namespace std;
int main()
{
	int a[3];
	char *ptr1,*ptr2;
	ptr1=(char*)&a[0];
	ptr2=(char*)&a[1];
	cout<<ptr2-ptr1;
	return 0;
}
#include<iostream>
using namespace std;
int main()
{
	int a[3];
	char *ptr1,*ptr2;
	ptr1=(char*)&a[0];
	ptr2=(char*)&a[1];
	cout<<ptr2-ptr1;
	return 0;
}

<< moderator edit: added [code][/code] tags >>

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Re: substitute of sizeof operator

Sep 6th, 2005

not guaranteed to work for all types. Some it will, some it wont. Theres no standard way of doing this. Its no coincidence that sizeof is a keyword.

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Re: substitute of sizeof operator

Sep 6th, 2005

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Originally Posted by Stoned_coder

not guaranteed to work for all types. Some it will, some it wont. Theres no standard way of doing this. Its no coincidence that sizeof is a keyword.

Could you elaborate, please?

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Re: substitute of sizeof operator

Sep 6th, 2005

Well actually now I begin to feel as if I was wrong. I didnt think you were allowed arrays of unions. But it actually seems you are. So as arrays are always contiguous it should actually work.
In 15 years I have never seen an array os unions. I thought they were prohibitted.

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Re: substitute of sizeof operator

Sep 6th, 2005

also any type with a user defined address of operator may cause trouble. Not impossible to get round but you wont be able to do it as above exactly.

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Re: substitute of sizeof operator

Sep 6th, 2005

seems to work with c++ STL classes too

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#pragma warning(disable: 4786)
#include <iostream>
#include <vector>
#include <string>
using namespace std;
 
int main()
{
	vector<string> array;
	array.resize(2);
	char* p1 = (char *)&array[0];
	char* p2 = (char *)&array[1];
	cout << p2-p1 << endl;
	cout << sizeof(array[0]) << endl;
	return 0;
}
#pragma warning(disable: 4786)
#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main()
{
	vector<string> array;
	array.resize(2);
	char* p1 = (char *)&array[0];
	char* p2 = (char *)&array[1];
	cout << p2-p1 << endl;
	cout << sizeof(array[0]) << endl;
	return 0;
}

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Re: substitute of sizeof operator

Sep 6th, 2005

try with this class.....

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 C++ Syntax (Toggle Plain Text) 
class hiddenaddr
{
   public:
      void* operator &()const { return NULL;}
};
class hiddenaddr
{
   public:
      void* operator &()const { return NULL;}
};

or with this one

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
class noaddrop
{
   private:
      void* operator &()const;
};
class noaddrop
{
   private:
      void* operator &()const;
};

For extra kudos (not open to Narue.... she has kudos in abundance already

) try to work out how to circumvent these operator overloads so that sizeof can be simulated for these classes too.

Last edited by Stoned_coder; Sep 6th, 2005 at 11:03 pm. Reason: fix ma speling and const correctness was forgotten in a stoned daze

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Re: substitute of sizeof operator

#10

Sep 7th, 2005

If you use an actual array, &array[0] is just a weird way of saying array.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
class hiddenaddr
{
   public:
      void* operator &()const { return NULL;}
};
 
class noaddrop
{
   private:
      void* operator &()const;
};
 
int main() {
 
	hiddenaddr x[1];
	std::cout << ((char *)(x + 1)) - ((char *) x) << std::endl;
 
	noaddrop y[1];
	std::cout << ((char *)(y + 1)) - ((char *) y) << std::endl;
 
	return 0;
}
#include <iostream>

class hiddenaddr
{
   public:
      void* operator &()const { return NULL;}
};

class noaddrop
{
   private:
      void* operator &()const;
};

int main() {

	hiddenaddr x[1];
	std::cout << ((char *)(x + 1)) - ((char *) x) << std::endl;

	noaddrop y[1];
	std::cout << ((char *)(y + 1)) - ((char *) y) << std::endl;

	return 0;
}