boost::variant get last accessed type

This is what I want to do:

boost::variant a<int, string>;
int b;
a=4;
b=a; //doesn't work. What is the easiest way to make b=4?

I 开发者_运维问答know I can use get, but I want to be able to do this without specifying the type. I can do it with apply_visitor and a visitor object, but I was wondering if there is a simpler way.

If you have a compiler that supports C++0x, you can use the amazing decltype:

boost::variant a<int, string>;
int b;
a = 4;
b = boost::get<decltype(b)>(a);

I don't know why you'd want to do this though since you already know the type.

You could write a helper function:

template <class V, typename T>
copy_variant(const V& v, T& t) { t = get<T>(v); }

// ...

copy_variant(a, b);

But seriously, I think this costs you more than it buys you.

Nope.

You can call variant<>::which() to get the index of the variant<>'s currently initialized type or variant<>::type() to get the std::type_info for the currently initialized type, but there's no way to extract the value of the currently initialized type other than get<> and apply_visitor.