Problem in G++: invalid C++ class members offsets

[Ilowry]

Hello everybody!
I've encountered a very strange problem while compiling the
AutoDetectProxy unittest in libjingle for iOS.
It looks like the compiler calculates wrong offsets for members of the
AutoDetectProxy class.

The AutoDetectProxy class is defined like this:

class AutoDetectProxy : SignalThread
{
... // Methods declarations and definitions;
     std::string agent_;  // 1st member of AutoDetectProxy
     std::string  server_url_;
};

In GDB 'p /x sizeof (SignalThread)' prints 0x188.
If you are inside of AutoDetectProxy constructor then 'p /x (int)&this-
>agent_ - (int)this' will also print 0x188.
And 'p /x sizeof(*this)' prints 0x1d4.

But if you outside of AutoDetectProxy code, exactly in
AutoDetectProxyTest::Create() method that dynamically allocates
AutoDetectProxy object and stores the pointer into auto_detect_proxy_
member,
then 'p /x (int)&auto_detect_proxy_->agent_ - (int)auto_detect_proxy_'
will give you 0x180!
And p /x sizeof(*auto_detect_proxy_) will print 0x1cc.

And in the same time 'p /x sizeof (SignalThread)'  and 'p /x
sizeof(*(SignalThread*)auto_detect_proxy_)' still print 0x188.

I'm puzzled and have no idea how to fix this. :(
Any ideas, please? This is in llvm gcc 4.2 on Mac OS X.