'sizeof' a class

Continuation from last week. What is the sizeof of a class? Wrote a simple program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

class A
{
  int a;
};

class B
{
  int b;
  int someFunc(void) {return b;}
};

class C
{
  void someFunc1(void) {};
  void someFunc2(void) {};
};

class D : public A
{
  void someFunc(void) {};
};


int main()
{
  cout<<"Size of class A = "<<sizeof(A)<<endl;
  cout<<"Size of class B = "<<sizeof(B)<<endl;
  cout<<"Size of class C = "<<sizeof(C)<<endl;
  cout<<"Size of class D = "<<sizeof(D)<<endl;
}

The output is as follows:

After finishing this, I found this interesting article here.

Checking the 'sizeof' doubts

I noticed sometime back that one of my programs behaved unexpectedly in certain scenarios which was traced to an incorrect use of sizeof. As a result, I made a small program to make sure that my understanding of sizeof is correct. Here is the program:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

int main()
{
 char a[]="";
 cout<<"Size of a[] = "<<sizeof(a)<<endl;

 char b[]=" ";
 cout<<"Size of b[ ] = "<<sizeof(b)<<endl;

 char c[10];
 cout<<"Size of c[10] = "<<sizeof(c)<<endl;

 char* d;
 cout<<"Size of d = "<<sizeof(d)<<endl;

 char* e = c;
 cout<<"Size of e = "<<sizeof(e)<<endl;

 char f[] = "123456";
 cout<<"Size of f = "<<sizeof(f)<<endl;

 char g[] = {'1','2','3','4','5','6'};
 cout<<"Size of g = "<<sizeof(g)<<endl;

 return 0;
}

The output is as follows:

A lot of interviewers love the sizeof questions.

Have you come across any interesting sizeof problems? Please feel free to add them in comments.

The size_t confusion

Apparently there could be a lot of confusion with regards to size_t. In 'C' the size_t was defined as an unsigned integer. 'C++' defines it as unsigned integer as well but it has a namespace of std around it. Some compilers may want #include<cstdlib> to use std::size_t.

In any case, it can cause quite some confusion among programmers as to why it is used and what is the size of it. Instead we can #define a type, give it a meaningful name and use it accordingly. Here is my simple example:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>
#include<cstdlib> //Used for std::size_t ... on some compilers

#define u16 unsigned short
#define u32 unsigned //integer is implicit

int main()
{
  size_t size = sizeof(size_t);
  std::cout << "Size of size_t is " << size << std::endl;

  std::size_t size2 = sizeof(std::size_t);
  std::cout << "Size of std::size_t is " << size2 << std::endl;

  size_t size3 = sizeof(u16);
  std::cout << "Size of u16 is " << size3 << std::endl;

  size_t size4 = sizeof(u32);
  std::cout << "Size of u32 is " << size4 << std::endl;

  return 0;
}

The output is as follows: