incomplete type in GDB for inner class with virtual methods defined in a separate file

Question

I have the problem with gdb 7.6 and g++ 4.8.3. I have the following 3 files

main.h

class A {
public:
    class B {
    public:
        virtual ~B();
        virtual void f();
        int abc;
    };
};

b.cpp

#include "main.h"

A::B::~B() {}

void A::B::f() {}

main.cpp

#include "main.h"

int main()
{
    int a=0;
    A::B x;
    A::B *y = &x;
    a = 10;
    return a;
}

Then

>> g++ main.cpp b.cpp -o main -g
>> gdb ./main
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /tmp/gdb/main...done.
(gdb) b 8
Breakpoint 1 at 0x400654: file main.cpp, line 8.
(gdb) r
Starting program: /tmp/gdb/main 

Breakpoint 1, main () at main.cpp:8
8       a = 10;
(gdb) p *y
$1 = <incomplete type>

We do not have the problem if

the definition of those methods is in main.h.
the methods are virtual.

Is this a known issue? How to workaround the problem?

The link suggests -g. I used -g in my case, still got the same error. — Joe C, Mar 10 '17 at 16:30
Yes, but in your case class `B` is inner. Looks like this makes the difference. — ks1322, Mar 10 '17 at 17:28

score 1 · Accepted Answer · answered Mar 10 '17 at 15:24

1

Looks like this is gdb bug. I also failed to print this variable with gdb 7.12, but lldb debugger was able to print it:

$ lldb ./main 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named lldb.embedded_interpreter
(lldb) target create "./main"
Current executable set to './main' (x86_64).
(lldb) b 8
Breakpoint 1: where = main`main + 36 at main.cpp:8, address = 0x000000000040068a
(lldb) r
Process 9542 launched: './main' (x86_64)
Process 9542 stopped
* thread #1: tid = 9542, 0x000000000040068a main`main + 36 at main.cpp:8, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x000000000040068a main`main + 36 at main.cpp:8
   5        int a=0;
   6        A::B x;
   7        A::B *y = &x;
-> 8        a = 10;
   9        return a;
   10   }
(lldb) p *y
(A::B) $0 = (abc = 0)

answered Mar 10 '17 at 15:24

ks1322

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Does lldb work with non-LLVM compiler? My code is compiled only by gcc in linux. – Joe C Mar 10 '17 at 19:00
1

Yes, I used gcc 6.2 + lldb here. In general you can almost always use gcc/clang with gdb/lldb, see http://stackoverflow.com/q/21132194/72178. – ks1322 Mar 10 '17 at 20:21
which emacs plugin do you use with lldb? – Joe C Mar 11 '17 at 03:12
I don't use emacs, I runned lldb directly. – ks1322 Mar 11 '17 at 10:05

incomplete type in GDB for inner class with virtual methods defined in a separate file

1 Answers1