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 is 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()` and pass a default value:
+
+```C++
+base::Optional<int> opt;
+opt.value_or(42); // will return 42
+```
+
+It is possible to initialize a `base::Optional<T>` from its constructor and
+`operator=` using `T` or 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
+opt_1 == base::nullopt_t; // false
+```
+
+`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 value. 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. For example, using -1 as the
+undefined value when the expected value can't be negative.
+
+## When not to use?
+
+It is recommended to not use `base::Optional<T>` as a function parameter as it
+will force the callers to use `base::Optional<T>`. Instead, it is recommended to
+keep using `T*` for arguments that can be ommited, with `nullptr` representing
+no value.
+
+Furthermore, depending on `T`, MSVC might fail to compile code using
+`base::Optional<T>` as a parameter because of memory alignment issues.