Solved

How to intepret output from gcc fdump-class-hierarchy option

Posted on 2012-04-11
2
1,028 Views
Last Modified: 2012-04-11
I have a simple class and wanted to study how it is organized into memory.
How can I interpret the following which was produced by using -fdump-class-hierarchy option in gcc.

Basically, I want to know where the vptr will be located in relation to the array for example.

Vtable for A
A::_ZTV1A: 3u entries
0     (int (*)(...))0
4     (int (*)(...))(& _ZTI1A)
8     A::SendCommand

Class A
   size=48 align=4
   base size=48 base align=4
A (0x223a600) 0
    vptr=((& A::_ZTV1A) + 8u)

Open in new window


class A
{
	public:
		A() { cout << "A's Constructor" << endl; };
		virtual void SendCommand(){};
		void setArray()
		{
			for(int i =0; i < 10; i++ )
			{
				m_array[i] = 'A';
			};
		};
	private:
		int m_a;
		int m_array[MAX_ARRAY];
};

Open in new window

0
Comment
Question by:ambuli
2 Comments
 
LVL 53

Accepted Solution

by:
Infinity08 earned 500 total points
ID: 37833826
>> How can I interpret the following which was produced by using -fdump-class-hierarchy option in gcc.

Vtable for A                            # first the vtable for the A class is listed
A::_ZTV1A: 3u entries                   # there are 3 entries in the vtable (aka _ZTV1A)
0     (int (*)(...))0                   # the first entry is always 0
4     (int (*)(...))(& _ZTI1A)          # the second entry is always a pointer to the typeinfo (aka _ZTI1A)
8     A::SendCommand                    # next follow entries for each of the virtual functions

Class A                                 # some more specifics about the A class
   size=48 align=4                      # the total size of an object is 48 bytes, with alignment on 4-byte boundaries
   base size=48 base align=4            # same information, but without taking alignment into account
A (0x223a600) 0                         # the pointer to the vtable can be found at offset 0 of an object
    vptr=((& A::_ZTV1A) + 8u)           # the virtual functions in the vtable can be found at offset 8 into the vtable

Open in new window



>> Basically, I want to know where the vptr will be located in relation to the array for example.

For gcc, the vptr is always in the first word of the object.
0
 

Author Closing Comment

by:ambuli
ID: 37834063
Great! Thank you.
0

Featured Post

Master Your Team's Linux and Cloud Stack

Come see why top tech companies like Mailchimp and Media Temple use Linux Academy to build their employee training programs.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Using 'screen' for session sharing, The Simple Edition Step 1: user starts session with command: screen Step 2: other user (logged in with same user account) connects with command: screen -x Done. Both users are connected to the same CLI sessio…
I. Introduction There's an interesting discussion going on now in an Experts Exchange Group — Attachments with no extension (http://www.experts-exchange.com/discussions/210281/Attachments-with-no-extension.html). This reminded me of questions tha…
The viewer will learn how to pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.
The viewer will learn how to user default arguments when defining functions. This method of defining functions will be contrasted with the non-default-argument of defining functions.

792 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question