Issues on getting the reference of the held value of boost::any

From: Seishi Onishi looper2k_at_[hidden])
Date: 2002-12-16 21:27:27

I'm using a collection of boost::any for the variable container class 
which has this map member.

std::map<std::string, boost::any> m_collection;

As you imagine, the container class can keep anything even the 
container class itself recursively. You know, an boost::any may hold 
copy expensive object. So I need a method to get the reference of the 
value held in boost::any not to make the expensive copy. But I found 
that the current any_cast implementation was not well support the 
reference.

Issue 1: The following code causes compiler error on VC6&7.
 boost::any dany = double(1.2);
 double& rd = boost::any_cast<double>(dany);

Issue 2: The following code works but one copy constructer called.
 class Foo foo;
 boost::any fany( foo );
 class Foo& rfoo = boost::any_cast<class Foo>(fany);

Actually, I found the way to sneak out of the issues.
 double& rd = *boost::any_cast<double>(&dany);
 class Foo& rfoo = *boost::any_cast<class Foo>(&fany);

But the problem is these codes won't throw bad_any_cast.
And I also wonder this *any_cast usage may not be the way the 
developer supposed. So I expect that the current any_cast code will 
be changed to well support the reference access.

It can be,
template<typename ValueType>
const ValueType& any_cast(const any & operand)
{
const ValueType * result = any_cast<ValueType>(&operand);
if(!result)
throw bad_any_cast();
return *result;
}

template<typename ValueType>
ValueType& any_cast(any & operand)
{
ValueType * result = any_cast<ValueType>(&operand);
if(!result)
throw bad_any_cast();
return *result;
}

-Seishi