[boost] Boost::any operator==

17 Apr 2006

      Hi
It would be nice if operator== could be added to boost::any. A partial
solution would be the following:

template<typename ValueType>
bool operator==(const ValueType & rhs) const
{
  return content ? content->equal(any(rhs).content) :
                   content == any(rhs).content;
}

bool operator==(const any &rhs) const
{
  return content && rhs.content ?
             content->equal(rhs.content) :
             content == rhs.content;
}

equal is a pure virtual member of placeholder declared as:

virtual bool equal(const placeholder *rhs) const = 0;

and implemented as (in holder):

virtual bool equal(const placeholder *rhs) const
{
  return type() == rhs->type() &&
    held == static_cast<const holder<ValueType>*>(rhs)->held;
}

However, this will break existing code when the holded object class does no
have operator== defined.
Kevlin Henney proposed:

namespace equality
{
  template<typename Type>
  bool operator==(const Type &lhs, const Type &rhs)
  {
    return &lhs == &rhs;
  }
}

virtual bool equal(const placeholder *rhs) const
{
  using namespace equality;
  return type() == rhs->type() &&
    held == static_cast<const holder<ValueType>*>(rhs)->held;
}

Thus providing a default operator== for everyone not having its own, which
returns true if the object addresses are the same. This will prevent
compiler errors when the operator== really needed to be there.
What I think is needed is:
1. If operator== is not used there should be no need for holded classes to
implement it.
2. If operator== is used, and the holded class does not define it, the code
must not compile.

Do you think this is possible to implement?

Thanks in advance
Jorge Lodos