CODEWITHSUNDEEP
c++classpointersstructthis
Usiten
Usiten

Reputation: 15

In-class struct, how does "this" work?

As stated, how does the this pointer act when called in a struct defined in a class ? Let's say I have the following piece of code:

class A {
public:
    struct {
        void bar() {  std::cout << this << std::endl;  }
    } foo;
};

int main() {
    A a; a.foo.bar();
    std::cout << &a << std::endl;
    std::cout << &a.foo << std::endl;
}

That generate this output:

0x62fe9f
0x62fe9f
0x62fe9f

a.foo share the same address with a, how could one access to the this pointer of foo ?

Using this->foo raise an error:

test.cpp:20:23: error: 'struct A::<anonymous>' has no member named 'foo'

Upvotes: 0

Views: 106

Answers (2)

Hayt
Hayt

Reputation: 5370

The address is mostly just where the memory if the object "starts". How much to offset is needed members is then defined by the class definition.

So class A "starts" at 0x62fe9f.

At the beginning of class A is the member foo so because there is nothing in front of it it has also address 0x62fe9f etc.

When you change the class to 

class A {
public:
    int i;
    struct {
        void bar() {  std::cout << this << std::endl;  }
    } foo;
};

You shloud see &a.i and &a having the same address and &a.foo should be &a + sizeof(int)

(Note: It may be more than sizeof(int) and in other cases also different because how padding is set. This is just a simple example. You should not rely on this in real code)

Upvotes: 6

zwol
zwol

Reputation: 140609

Possibly this modified version of your program, and its output, will be enlightening.

#include <iostream>
using std::cout;

class A {
public:
  int data;

  struct {
    int data;
    void bar()
    {
      cout << this << '\n';
      cout << &this->data << '\n';
    }
  } foo;

  void bar()
  {
    cout << this << '\n';
    cout << &this->data << '\n';
    cout << &this->foo << '\n';
  }
};

int main(void)
{
  A a;
  cout << &a << '\n';
  a.bar();
  a.foo.bar();
}

Upvotes: 3

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