add a value_or_eval similar to boost::optional

[malcolmdavey]

Hi TL
Thanks for your great library.

Was wondering if you had considered a value_or_eval, similar to the existing value_or, which returns the current value if it exists, or returns value from a lambda etc, but only executes if the optional is empty.

The point is to enable lazy evaluation, especially for expensive calculations.

(This exists in boost::optional, but not sure if there was a reason not to include it in the std.)

Thanks.

Here is a implementation from boost 1.61 for example

    template <typename F>
    value_type value_or_eval ( F f ) const&
      {
        if (this->is_initialized())
          return get();
        else
          return f();
      }
      
    template <typename F>
    value_type value_or_eval ( F f ) &&
      {
        if (this->is_initialized())
          return boost::move(get());
        else
          return f();
      }

[BenFrantzDale]

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.

[malcolmdavey]

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.

[hadrielk]

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