Base: add Optional<T>.

docs/optional.md

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+# base::Optional
+
+`base::Optional<T>` is a container that might contain an instance of `T`.
+
+[TOC]
+
+## History
+
+[base::Optional<T>](https://code.google.com/p/chromium/codesearch#chromium/src/base/optional.h)
+is an implementation of [std::optional<T>](http://en.cppreference.com/w/cpp/utility/optional),
+initially a C++ experimental feature and now part of the C++17 standard. The Chromium's
+implementation is as close as possible to the specification. The differences are listed
+at the beginning of the header. The most important difference is that all the objects and
+types are part of the `base::` namespace instead of `std::`. Also, following Chromium coding
+style, the class in named `Optional` instead of `optional`.
+
+## API description
+
+For a deep API description, please have a look at [std::optional<T>](http://en.cppreference.com/w/cpp/utility/optional)
+or the [Chromium implementation](https://code.google.com/p/chromium/codesearch#chromium/src/base/optional.h).
+
+When initialized without a value, `base::Optional<T>` will be empty. When empty, the `operator bool`
+will return `false` and `value()` should not be called. An empty `base::Optional<T>` is equal to
+`base::nullopt_t`.
+```C++
+base::Optional<int> opt;
+opt == true; // false
+opt.value(); // illegal, will DCHECK
+opt == base::nullopt_t; // true
+```
+
+To avoid calling `value()` when an `base::Optional<T>` is empty, instead of doing checks, it is
+possible to use `value_or()` if a default value is available:
+```C++
+base::Optional<int> opt;
+opt.value_or(42); // will return 42
+opt == base::nullopt_t; // false
+```
+
+It is possible to initialize a `base::Optional<T>` from its constructor and `operator=` using
+another `base::Optional<T>`:
+```C++
+base::Optional<int> opt_1 = 1; // .value() == 1
+base::Optional<int> opt_2 = base::Optional<int>(2); // .value() == 2
+```
+
+All basic operators should be available on `base::Optional<T>`: it is possible to compare a
+`base::Optional<T>` with another or with a `T` or `base::nullopt_t`.
+```C++
+base::Optional<int> opt_1;
+base::Optional<int> opt_2 = 2;
+
+opt_1 == opt_2; // false
+opt_1 = 1;
+
+opt_1 <= opt_2; // true
+opt_1 == 1; // true
+```
+
+`base::Optional<T>` has a helper function `make_optional<T&&>`:
+```C++
+base::Optional<int> opt = make_optional<int>(GetMagicNumber());
+```
+
+Finally, `base::Optional<T>` is integrated with `std::hash`, using `std::hash<T>` if it is not empty,
+a default value otherwise. `.emplace()` and `.swap()` can be used as members functions and `std::swap()`
+will work with two `base::Optional<T>` objects.
+
+## How is it implemented?
+
+`base::Optional<T>` is implemented using `base::AlignedMemory`. The object doesn't behave
+like a pointer and doesn't do dynamic memory allocation. In other words, it is guaranteed
+to have an object allocated when it is not empty.
+
+## When to use?
+
+A very common use case is for classes and structures that have an object not always available,
+because it is early initialized or because the underlying data structure doesn't require it.
+It is common to implement such patterns with dynamically allocated pointers, `nullptr`
+representing the absence of the data. Other approaches involve `std::pair<T, bool>` where
+bool represents whether the object is actually present. It can also be used for simple types,
+for example when a structure wants to represent whether the user or the underlying data
+structure has some value unspecified, a `base::Optional<int>` would be easier to understand
+than a special value representing the lack of it.
+
+## When not to use?
+
+TODO: list cases and explain why.

[danakj, 2016/04/14 20:16:20]

Talked a bit with jyasskin about this, I think the one thing we should point out
is: "Don't use it as an argument to a function, as it forces the caller to use
an Optional<T>. Instead continue to do something like T* for arguments that can
be omitted."

[mlamouri (slow - plz ping), 2016/04/16 11:43:43]

Done. Also mentioned the MSVC issue.