boost::variant implementation
boost, boost-variant, c++
Solution
It works pretty much the way you described. Long story short:
It has an integer `which` that indicates which data type is used.
The storage is implemented using boost's `aligned_storage` which basically is a buffer of the maximum data size. (it is in a union, but for alignment purposes)
Finally, the visitor is indeed implemented with a `switch`, generated at compile time using macros to unroll for all type possibilities.
Problem
I'm just curious about `boost::variant`'s implementation. Does it work like this? Two members: - A number representing the currently stored type (i.e. 0 for the first template parameter, 1 for the second template parameter etc) - A union of all possible types (which of course the size of the largest). `apply_visitor()`: Has a `switch` statement on the number representing the currently stored type to call the correct overload (this would in the worse case be compiled as jump table so take constant time). I understand there's also there are a number of optimisations which can sure `boost::variant` does not need to dynamically allocate memory as detailed here, but I think I get these.