Re: C++0x: Is this "this-parameter" forwarder for function<void(???)> correct?

Niels Dekker - no reply address <>
Sat, 25 Dec 2010 17:37:43 +0100
On 2010-12-23 7:25 PM, Johannes Schaub (litb) wrote:

I read some discussion on how to best make
     struct Class { void f() { } };
     function<void(???)> f = bind(&Class::f, _1);
     Class c;
work with both

And I thought the following should work

template<typename T> struct ThisParam {
   T *t;
   ThisParam(T *t):t(t) { }
   ThisParam(T&t):t(&t) { }

   operator T*() const { return t; }
   operator T&() const { return *t; }

function<void(ThisParam<Class>)> f = bind(&Class::f, _1);
Class a;

But unfortunately, GCC doesn't like this. I thought the "operator T*" is
used by INVOKE(...)'s ((*t1).*f)(...). I supplied the "operator T&" in case
the user binds a function that wants a "Class&" as argument.

But GCC doesn't like the code when it has the "operator T&" - it only
accepts when i remove it. And I also find it confusing that it rejects when
I instead define it as follows and use it on a bound member function

template<typename T> struct ThisParam {
   T *t;
   ThisParam(T *t):t(t) { }
   ThisParam(T&t):t(&t) { }

   T&operator *() const { return *t; }

Why is this? Do I misread 20.8.2[func.require] or is GCC's C++0x standard
library broken?

FWIW, I tried your code on both MSVC 2010 and GCC 4.5.1 20100924 (Red
Hat 4.6.1-4), after having added the necessary #include <functional> and
using-declarations: It compiled well on
MSVC 2010, but it failed on GCC 4.5.1 (g++ -std=c++0x). Just like you're
saying. :-(

My 2 cents, Niels
Niels Dekker
Scientific programmer at LKEB,
Leiden University Medical Center

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