Why is the size of an empty class not zero in C++?

 

 

　　Predict the output of the following program?

#include<iostream>
using namespace std;
 
class Empty 
{
};
 
int main()
{
  cout << sizeof(Empty) << endl;
  return 0;
}

 　　Output：　　1

　　

　　Size of an empty class is not zero. It is 1 byte generally. It is non-zero to ensure that the two different objects will have different addresses.

　　See the following example.

#include<iostream>
using namespace std;

class Empty 
{
};

int main()
{
    Empty a, b;
    
    if (&a == &b)
    {
        cout << "impossible " << endl;
    }
    else
    {
        cout << "Fine " << endl;
    }
    
    return 0;
}

　　Output: 　　Fine

 

　　For the same reason (different objects should have different addresses), “new” always returns pointers to distinct objects.
　　See the following example.

#include<iostream>
using namespace std;
 
class Empty 
{
};
 
int main()
{
    Empty* p1 = new Empty;
    Empty* p2 = new Empty;
 
    if (p1 == p2)
    {
        cout << "impossible " << endl;
    }
    else
    {
        cout << "Fine " << endl;
    }
    return 0;
}

　　Output：　　Fine

　　Now guess the output of following program (This is tricky).

#include<iostream>
using namespace std;
 
class Empty 
{
};
 
class Derived: Empty 
{ 
    int a; 
};
 
int main()
{
    cout << sizeof(Derived);
    return 0;
}

　　Output (with GCC compiler):  　　4

　　

　　Note that the output is not greater than 4. There is an interesting rule that says that an empty base class need not be represented by a separate byte. So compilers are free to make optimization in case of empty base classes.

　　As an excercise, try the following program on your compiler.

#include <iostream>
using namespace std;
  
class Empty
{
};
  
class Derived1 : public Empty
{
};
  
class Derived2 : virtual public Empty
{
};
  
class Derived3 : public Empty
{    
    char c;
};
  
class Derived4 : virtual public Empty
{
    char c;
};
  
class Dummy
{
    char c;
};
  
int main()
{
    cout << "sizeof(Empty) " << sizeof(Empty) << endl;
    cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
    cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;
    cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;
    cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;    
    cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;
  
    return 0;
}

　　运行结果如下所示。

　　

 

　　补充：

　　运行如下代码，运行结果为：　　1

#include    <cstdio>
#include    <iostream>
using namespace std;
class Test
{
    int arr[0];
};

int main()
{
    Test t;
    cout<<sizeof(t);
    return 0;
}

 

 

 

 

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　　2013-11-25　　20:39:10