Size of C++ classes

Question

Here is the code which prints size of different classes

#include <iostream>

using namespace std;

class EmptyClass
{    
};

class AbstractClass
{
  public: 
          virtual void funcOne() = 0;
          virtual void funcTwo() = 0;
};

class NotAbstrClass
{
  public: int virtFunc( int );
};

class MixClass
{
  public:
          virtual void clFunc( int );
          static int i;
          int j;
};

int main()
{
    // Print size of class or class objects
    cout<<"Size of empty class: "<< sizeof(EmptyClass)<<endl;          
    cout<<"Size of Abstract class :"<< sizeof(AbstractClass)<<endl;
    cout<<"Size of Non Abstract class: "<< sizeof(NotAbstrClass)<<endl;
    cout<<"Size of Mix class: "<< sizeof(MixClass)<<endl;
    return 0;
}

The output of the program on C++11 compiler is

Size of empty class: 1
Size of Abstract class :4
Size of Non Abstract class: 1
Size of Mix class: 8

I understand why Empty class has size 1 Size of empty class object. For abstract class, the object stores a pointer for implementing virtual function call mechanisms. But what about the sizes of other class objects (NotAbstrClass and MixClass) ?

Answer 1

According to Girish Shetty:

There are many factors that decide the size of an object of a class in C++.

These factors are:

• Size of all non-static data members
• Order of data members
• Byte alignment or byte padding
• Size of its immediate base class
• The existence of virtual function(s) (Dynamic polymorphism using virtual functions).
• Compiler being used
• Mode of inheritance (virtual inheritance)

Here there are some related website, I think it can be helpful to you.

Determine the size of class object: http://www.cprogramming.com/tutorial/size_of_class_object.html

Memory layout: http://www.phpcompiler.org/articles/virtualinheritance.html

And, if you use MVSC, you can dump all memory layout of all class in your solution with -d1reportAllClassLayout like that:

cl -d1reportAllClassLayout main.cpp

Answer 2

NotAbstrClass has no data members, so it too is an empty class. Since classes cannot be zero-sized, you get the same treatment as EmptyClass.

MixClass has a virtual function, and 1 non-static data member. It seems each of these (vptr and int) occupy 4 bytes on your platform, so the size is 8 bytes.

Answer 3

NotAbstrClass is like an empty class when we talk about bit sizes since it has no data. MixClass has the virtual function pointer (4 bytes on a 32-bit machine) and an int (also 4 bytes).