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- [section Type requirements]
- The very minimum requirement of `optional<T>` is that `T` is a complete type and that it has a publicly accessible destructor. `T` doesn't even need to be constructible. You can use a very minimum interface:
- optional<T> o; // uninitialized
- assert(o == none); // check if initialized
- assert(!o); //
- o.value(); // always throws
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- But this is practically useless. In order for `optional<T>` to be able to do anything useful and offer all the spectrum of ways of accessing the contained value, `T` needs to have at least one accessible constructor. In that case you need to initialize the optional object with function `emplace()`, or if your compiler does not support it, resort to [link boost_optional.tutorial.in_place_factories In-Place Factories]:
- optional<T> o;
- o.emplace("T", "ctor", "params");
- If `T` is __MOVE_CONSTRUCTIBLE__, `optional<T>` is also __MOVE_CONSTRUCTIBLE__ and can be easily initialized from an rvalue of type `T` and be passed by value:
- optional<T> o = make_T();
- optional<T> p = optional<T>();
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- If `T` is __COPY_CONSTRUCTIBLE__, `optional<T>` is also __COPY_CONSTRUCTIBLE__ and can be easily initialized from an lvalue of type `T`:
- T v = make_T();
- optional<T> o = v;
- optional<T> p = o;
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- If `T` is not `MoveAssignable`, it is still possible to reset the value of `optional<T>` using function `emplace()`:
- optional<const T> o = make_T();
- o.emplace(make_another_T());
- If `T` is `Moveable` (both __MOVE_CONSTRUCTIBLE__ and `MoveAssignable`) then `optional<T>` is also `Moveable` and additionally can be constructed and assigned from an rvalue of type `T`.
- Similarly, if `T` is `Copyable` (both __COPY_CONSTRUCTIBLE__ and `CopyAssignable`) then `optional<T>` is also `Copyable` and additionally can be constructed and assigned from an lvalue of type `T`.
- `T` ['is not] required to be __SGI_DEFAULT_CONSTRUCTIBLE__.
- [endsect]
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