Subject: Re: [Boost-users] can't extend boost::any on windows
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-12-16 13:49:34
AMDG
Michael Linck wrote:
> Hi, I've implemented a simple, low level json formatting utility to
> complement tinyjson's lightweight parser. To do this, I've extended
> boost::any only to add a "format()" function to it that is aware of
> what types the content can take in a json structure and produces the
> appropriate string. The default constructor and the constructor with
> a value are the only things I implemented, aside from the format
> function. Both just call the base constructor. Compiled in gcc,
> everything works fabulously. Apparently, a functioning assignment
> operator and copy constructor are appropriately inherited and
> everything runs great. On windows, however, some subtle difference
> causes assignment and copies to seg-fault. The real problem is that,
> for the life of me, I can't find a way to implement assignment or
> copies explicitly, that doesn't also bomb out. I can probably come up
> with a work around that stores my type in shared pointers so they're
> never actually copied, but I'm really curious if anyone could explain
> what causes this. Here are some code snippets to illustrate the
> behavior:
>
> <snip>
>
> I'm not going to list the different ways I've attempted to implement
> copy and assignment, some more plausible than others, out of respect
> for brevity. If you try a build on windows, VC8, you should be able
> to mess around with this to your hearts content. It's really annoying
> because it was a factor in the fundamental elegance of the parsing
> implementation, because I see no reason why it shouldn't work, because
> it worked fine in linux, and because it put my windows port at least a
> day behind customer expectations.
msvc gets the copy constructor and assignment operator wrong in
the presence of a templated base class constructor/assignment operator.
You need to define them explicitly.
class Value : public boost::any {
public:
// <snip>
Value(const Value& arg) : boost::any(static_cast<const
boost::any&>(arg)){}
Value& operator=(const Value& arg) {
static_cast<Value&>(*this) = static_cast<const boost::any&>(arg);
}
};
In Christ,
Steven Watanabe