On Wed, May 25, 2011 at 11:12:02AM -0700, Ian Lance Taylor wrote:
> "Frank B. Brokken" <f.b.brokken@xxxxxx> writes:
>
> > Dear Ian Lance Taylor, you wrote:
> >
> >> This approach will only work if you guarantee that every piece of code
> >> that creates a new MultiStreambuf includes that header file.
> >
> > I'm not sure if I follow you here. All MultiStreambuf sources include the
> > above header, and no other (external to MultiStreambuf) functions need/have
> > access to MultiStreambuf's private members.
> >
> > So I guess you do not mean by 'creates a new MultiStreambuf' constructions
> > like:
> > void fun()
> > {
> > MultiStreambuf ms;
> > MultiStreambuf *msp = new MultiStreambuf;
> > }
>
> That is pretty much exactly what I do mean, actually. There is some
> more information at
>
> http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Vague-Linkage.html
>
> Note specifically: "Make sure that any inline virtuals are declared
> inline in the class body, even if they are not defined there." You
> aren't currently doing that. It's possible that that by itself might
> fix this problem; I'm not sure.
Nevertheless, I'm also wondering also about another situation, which
might occur (or is it the same ?):
the function "overflow" is defined already as protected in the
base-class std::streambuf, from which MultiStreambuf is derived
(public).
Now, what would happen if (on armel):
- your MultiStreambuf-class and all derived classes ONLY call
"this->overflow" in situations where g++ can guarantee that "this" is
a MultiStreambuf, and thus inline the code
==> it is possible to generate the library without exporting
"MultiStreambuf::overflow"
(i'm not sure: would that be OK from G++, or a violation of the
standard?)
- but when the library is used, an object of type MultiStreambuf is
used through the std::streambuf - interface, such that a call to
overflow is generated -- which can't be satisfied due to the missing
exported function.
In fact, I tried a minimal example of this situation, where I also get
an error on amd64 (Debian, g++-4.4.5) with the following code:
base.h:
class Base{ // base class defining an inline virtual function as public
public:
inline virtual int operator()()const{ return 2; }
int get2()const{ return this->operator()(); }
virtual ~Base(){};
};
derived.h:
#include "base.h" // derived class, defining the function as private &
// using it only once
class Derived : public Base{
public:
~Derived(){}
int get()const;
private:
inline virtual int operator()()const;
};
derived.cc:
#include "derived.h" // separate compile unit, where the type of "this"
// is well-defined ==> operator() can be inlined
int Derived::get()const{ return this->operator()();}
int Derived::operator()()const{ return 4; }
test.cc:
#include "derived.h" // access the virtual function of Derived
// through "get2"
#include <iostream>
int main(){
Base *b = new Derived();
std::cout << b->get2() << std::endl;
}
Compiling with "g++ test.cc derived.cc" gives a warning:
warning: inline function âvirtual int Derived::operator()() constâ used but never defined
and a linker error:
/tmp/ccU781xY.o:(.rodata._ZTV7Derived[vtable for Derived]+0x10): undefined reference to `Derived::operator()() const'
Maybe in the MultiStreambuf-case something similar is happen?
Axel
