add a value_or_eval similar to boost::optional about optional HOT 3 OPEN

I like this. The other way I've thought of doing this is

template <typename Fn>
struct on_demand {
    Fn fn;
    template <typename T>
    operator T() const& { return fn(); }
    template <typename T>
    operator T() && { return std::move(fn)(); }
};

which lets you do opt.value_or(on_demand{[] { return doSomethingExpensive(); }}). I'm not sure which I like more, but there are other cases where an on_demand like this can be useful.

from optional.

Thanks - that looks like a neat work around. I guess it does depend on the value_or implementation not having any surprises, and does rely on people knowing the trick I guess.

from optional.

Since what you're asking for is something like value_or() with or_else(), a better function name might be value_else()?

from optional.