Class Memory Allocation Size

[DnshPly9]

Hello Friends,

I have a question for allocation of memory to classes in C++

Pls have a look at the code -

#include<iostream>
#include<conio.h>

using namespace std;

class A
{
      public:
      A()
      {
                }
      };
    
class B
{
      public:
      B()
      {
                }
      };
  
class C
{
      public:
             int a;
      C()
      {
                }
      };  
  
int main()
{
    A a1;
    cout << sizeof(a1) << endl;
    B *b1 = new B();
    cout << sizeof(b1) << endl;
    C c1;
    cout << sizeof(c1) << endl;
    getch();
    return 0;
}      

This code gives us output -
1
4
4

Now how is the memory allocated that in case of Class A it is only 1,
and in Class C it is 4.
And in case of Class B which is declared using new is also 4.

Pls help me understand this.

Thank You.

DnshPly9

[quirkyusername]

the reason sizeof(b1) is 4 is because b1 is a pointer, pointers are always 4 bytes regardless of what they point to. c1 is bigger in size that a1 because instance of C contain an integer member variable, whereas the A class is empty

[jsmith]

@quirkyusername: Pointers are not always 4 bytes.

[DnshPly9]

Hii,

Ok I thought @quirkyusername is right.
But here is contradiction too.

@jsmith can u help me by elaborating how does it work then..???
How is the memory allocated.
anyone...please...

Thanks

DnshPly9

[quirkyusername]

Ok well it's platform dependant but if you continue using the same platform pointers will always take up 4 bytes, my point was that the data type pointed to doesn't affect their size

[LB]

Still, why does sizeof(A) give 1?

[Bazzy]

Because it doesn't contain any member, and the minimum size for a type is 1
http://www2.research.att.com/~bs/bs_faq2.html#sizeof-empty

[jsmith]

Sigh.

@quirkyusername: Run this:

#include <iostream>

struct Foo {
    virtual void bar() {}
};

int main() {
    void (Foo::*fn)() f = &Foo::bar;
    std::cout << sizeof( f ) << std::endl;
}

I haven't run this myself, but with gcc I'm betting you get 6.

You need to read up on the standard about pointers. All pointers-to-POD types, yes,
might be the same size. But pointers-to-members, no, not necessarily.

[jsmith]

After fixing a compile error:

#include <iostream>

struct Foo {
    virtual void bar() {}
};

int main() {
    void (Foo::*fn)() = &Foo::bar;
    std::cout << sizeof( f ) << std::endl;
}

Output is:

8

[EDIT: I'm running 32-bit linux]

[EDIT2: Ok, this time I pasted the fixed code and not the original]

[ne555]

Your code doesn't compile. I change this:

//void (Foo::*fn)() f = &Foo::bar;
void (Foo::*f)() = &Foo::bar;
//Or
typedef void (Foo::*fn) ();
fn f = &Foo::bar;