boost::optional alternative in C++ Standard Library

Question

I'm trying to get my program working without boost usage, but can't find an alternative of some useful patterns. Namely, I can't find boost::optional-likewise pattern in the standard library. Is there some standard alternative for boost::optional (C++11 or somewhere else)?

Teheran

C++0x == C++11, removed from question. :) – Alonaalone

You can use a nullable pointer as a cheap and unsafe alternative to boost::optional. Or maybe std::unique_ptr, where the value 0 (null_ptr, NULL) is interpreted an uninitialized optional type. Besides the drawbacks the good thing is that you dont need to implement anything; but just keep the abstraction in mind. A clue about this is given in the boost manual itself boost.org/doc/libs/1_53_0/libs/optional/doc/html/boost_optional/… "As a result, you might be able to replace optional<T> by T* on some situations but not always" – Shirlyshiroma

Answer 1

Short answer: No.

Long answer: Roll your own according to the boost spec. The documentation is quite exhaustive and the code isn't that complex, but this still requires above average C++ skills.

To update this answer: C++14 unfortunately did not ship with std::optional. The current proposal (Revision 5) is N3793 and it is expected to be shipped as a separate technical specification or to become part of C++17.

Uncommunicative

In some environments, it is politically impossible to use boost, sadly. – Hystero

Have you thought about extracting just optional from boost? – Padus

Good answer, except for the ", but why would you try to get rid of boost anyway?" - Like others have posted, Boost isn't always feasible; not to mention, including such a massive library (and all the headaches of managing version dependencies) for one very light-weight feature is not ideal. – Sain

@Copperpot There you go. I actually removed that piece of opinion. – Uncommunicative

Personally, I'd recommend avoiding boost::optional. This is primarily because operator= assigns the stored object instead of destroying and copy/move constructing. It seems like a cool feature because it looks like it logically means the right thing but that's only true for an lvalue, not an xvalue - which is what is held by boost optional. en.cppreference.com/w/cpp/types/aligned_storage makes it very easy to roll your own that behaves correctly (it's just five or six lines) – Cana

@Cana I'm a little confused here, shouldn't the stored object's type have a suitable operator= defined for it that knows what to do in each case? – Overwrite

You can't have operator= on a constant object or a reference. So you can't even express such a program. The operator= of the instance of optional itself would need to be specialised for when there can be no correct operator= following sensible laws so that it remakes the stored object entirely. That might be confusing to some but okay for others. unfortunately N3793 section X.Y.4.3 has operator=(nullopt_t) which will be a problem for optional<optional<T>> - which is an important use case for implementing formally defined programs but those always need to compose sensibly – Cana

Update: std::optional is part of C++17. – Asinine

@pmr, please, update your question to include C++17. – Davisdavison

Answer 2

There is currently a proposal for C++14 (or C++17). So the answer is (probably) not yet :).

Padus

Apparently it has just been voted out C++14 into a Technical Specification (see en.cppreference.com/w/cpp/utility/optional) :( – Padus

Isn't it the case that 'voting out' in C++ committee-speak actually means 'voting for' ? – Flitch

Answer 3

Like pmr explained, it is not possible right now, and will not be until C++17 is out.

However, you should be able to use this single header library on github as a drop in replacement of boost- or std optional. It has no dependencies (except a c++11/c++14 capable compiler).

Conant