Boost C++ Libraries: Ticket #301: boost::function doesn't accept classes declared in functions

Vladimir Prus — Thu, 09 Sep 2004 13:59:33 GMT

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To begin with, it seems it's boost::ref which fails. I believe this will
never work, on any compiler, because local classes are not valid
template arguments. What do you think?

status changed

Douglas Gregor — Thu, 09 Sep 2004 14:03:09 GMT

status assigned → closed

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Vladimir is correct: local classes are not valid template arguments, so it
is impossible for boost::function (or boost::ref) to support this usage
(although I wish it could!).

nobody — Thu, 09 Sep 2004 14:12:47 GMT

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Thanks for the swift response and "ouch." :)
I had really been fantisising about local classes being
exactly equivalent to global/namespaced/nested ones. Thanks
again!
John Engstrom

ossmdw45 — Thu, 09 Sep 2004 14:52:04 GMT

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> Summary: boost::function doesn't accept classes declared
in functions
> This fails to compile, see below for error messages,
> however if I move the declaration of the functor out of
> the function bar(), it compiles fine. Perhaps it only
> happens with MSVC.
No, that's a limitation of C++ templates. Only
classes/structs with external linkage can be used as
template arguments, IIRC. I've heard people talking about
removing this restriction for specifically the reason you're
talking about: to be able to define "lambda" functions
in-scope. But right now it's illegal.
You _can_, however, define it this way (code not compiled
since I'm at work):
void bar()
{
   ...
   struct localfunction
   {
       static void foo(int arg1, int arg2) {
           moo();
       }
       ...
   };
   boost::function2<void, int, int> my_function1 =
boost::ref(localfunction::foo);
   boost::function2<void, int, int> my_function2 =
localfunction::foo;
}
This works (should work) because localfunction::foo is just
a regular function pointer (int*int->void), so you can
templatize on it.

nobody — Wed, 29 Sep 2004 15:14:02 GMT

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Yes! Cool technique Max !
With some macros, I was able to make boost::function<>
objects out of my local functors..
John Engstrom