C++17 in Detail: A Deep Dive/

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Default Construction

Default Construction

Using std::optional with class constructors can be slightly inefficient sometimes. In this lesson, we'll learn a way to fix that.

We'll cover the following...

Movable Types

If you have a class with a default constructor, like:

C++
class UserName {
  public:
  UserName() : mName("Default")
  {
  }
  // ...
};

How would you create an optional that contains UserName{}?

You can write:

C++
std::optional<UserName> u0; // empty optional
std::optional<UserName> u1{}; // also empty
// optional with default constructed object:
std::optional<UserName> u2{UserName()};

That works but it creates an additional temporary object. If we traced each different constructor and ...