result

std::variant with c++17, but on linux at least you need to install gcc from git to have access to it so far. boost::variant otherwise, but depending on the types used (if there aren't any that are nothrow default constructible) it allocates on assignment. If you just want a result type and not a more general "rust enum", this repository does it (while only allocating space for the larger, I think) https://github.com/oktal/result/ And I don't think there's any reason you couldn't implement an equivalent of std::variant for previous versions of C++, I'm just not aware of any.