[boost] Variant Library: top level const types

From: Gennadiy Rozental (gennadiy.rozental_at_[hidden])
Date: 2003-04-02 01:46:38

> > 2. Top level const requirement on bounded types.
> > It's unreasonable. I should be able to define variant with const
> > types. It will be as usable as usual constant types are. The only
> > requirements that we may incur is that if one types is const,
> > rest should be also.
>
> It's actually not unreasonable: one of the primary goals of variant is
> to match the semantics of its bounded types as closely as possible. The
> prohibition on top-level const types, then, is quite reasonable.
>
> To see, consider the following:
>
>   const int i = 3;
>   i = 4; // error
>
>   variant<const int> v;
>   v = 4; // error?
>
> If top-level const types *were* allowed, then the assignment would
> succeed. Itay and I decided such was highly undesirable. Let me know if
> you disagree with the above reasoning.

If I understood you properly you having problem with the following code
successful compilation:

template<typename T>
struct A
{
    char buf[4];

    template<typename Arg>
    void
    operator=( Arg const& arg )
    {
        new (buf) T(arg);
    }
};

int main() {
    A<const int> a;

    a = 4;
}

This wouldn't happened if you use reinterpret cast based implementation, but
probably do not want to rely on it. What I would do is to explicitly
prohibit some operations using static asserts for top level const type
situation. It wouldn't cost you anything and allow to support this usage
case.

Gennadiy.