TotW123.md

Tip of the Week #123: absl::optional and std::unique_ptr

类数据成员的包装属性，一般根据实际情况按照下面的优先级来使用：

1. 无包装形式
2. std::optional 包装形式
3. std::unique_ptr 包装形式

#include <memory>
#include "third_party/absl/types/optional.h"
#include ".../bar.h"

class Foo {
  ...
 private:
  Bar val_;
  absl::optional<Bar> opt_; // `std::optional` also works
  std::unique_ptr<Bar> ptr_;
};

三种类数据成员包装属性的对比情况：

	Bar	absl::optional	std::unique_ptr
Supports delayed construction		✓	✓
Always safe to access	✓
Can transfer ownership of Bar			✓
Can store subclasses of Bar			✓
Movable	If Bar is movable	If Bar is movable	✓
Copyable	If Bar is copyable	If Bar is copyable
Friendly to CPU caches	✓	✓
No heap allocation overhead	✓	✓
Memory usage	sizeof(Bar)	sizeof(Bar) + sizeof(bool)	sizeof(Bar*) when null, sizeof(Bar*) + sizeof(Bar) otherwise
Object lifetime	Same as enclosing scope	Restricted to enclosing scope	Unrestricted
Call f(Bar*)	f(&val_)	f(&opt_.value()) or f(&*opt_)	f(ptr_.get()) or f(&*ptr_)
Remove value	N/A	opt_.reset(); or opt_ = absl::nullopt;	ptr_.reset(); or ptr_ = nullptr;