Comments on: Dumping a C++ object’s memory layout with Clang

By: eliben

eliben — Sun, 13 Jan 2013 22:35:14 +0000

@Wilfred,

Good question – I noticed this also. Actually I don’t know the exact cases in which Clang prints out the layout – maybe it makes sense to ask in the cfe-dev list.

By: Wilfred

Wilfred — Sun, 13 Jan 2013 21:16:12 +0000

Hi Eli,

Very helpful post, thanks. Do you have any idea why “sizeof” is required in order to get the object’s layout? If the class is not instantiated with the “new” keyword, or its size is not checked in the code, its layout won’t be printed at all. For example, the following example succeeds:

struct s { bool b; };

int main(){
	s *p = new s;
	return 1;
}

The layout of the struct “s” is printed correctly. But this won’t work:

struct s { bool b; };

int main(){
	s o;
	s *op = &o;
	return 1;
}

I get no output at all from “clang -cc1 -fdump-record-layouts test.cpp” in this case. Is this a bug in the clang frontend?

By: eliben

eliben — Wed, 26 Dec 2012 21:52:47 +0000

@Eden,

Fixed, thanks.

By: Eden Cohen

Eden Cohen — Wed, 26 Dec 2012 14:28:24 +0000

Small typo (how can it be – isn’t this the output of clang ?):
In the first dump [int foo] is shown as [int f]

By: Marek

Marek — Wed, 19 Dec 2012 14:33:22 +0000

To make the output of VC readable use undname.exe to it.

By: Marek

Marek — Wed, 19 Dec 2012 14:09:54 +0000

If you are using Visual Studio, you can print the same using undocumented /d1reportAllClassLayout switch: `

class Derived   size(40):
        +---
        | +--- (base class Base)
 0      | | foo
        | +---
 4      | bar
 8      | baz
        | __abENT__lt;alignment member__abENT__gt; (size=4)
16      | Point a_point
32      | c
        | __abENT__lt;alignment member__abENT__gt; (size=7)
        +---

` and second example: `

class Derived   size(40):
        +---
        | +--- (base class Base)
 0      | | {vfptr}
 4      | | foo
        | +---
 8      | bar
12      | baz
16      | Point a_point
32      | c
        | __abENT__lt;alignment member__abENT__gt; (size=7)
        +---
Derived::$vftable@:
        | __abENT__amp;Derived_meta
        |  0
 0      | __abENT__amp;Derived::method

Derived::method this adjustor: 0