Effective Modern C++

Chapter 5. Rvalue References, Move Semantics, and Perfect Forwarding

When you first learn about them, move semantics and perfect forwarding seem pretty straightforward:

• Move semantics makes it possible for compilers to replace expensive copying operations with less expensive moves. In the same way that copy constructors and copy assignment operators give you control over what it means to copy objects, move constructors and move assignment operators offer control over the semantics of moving. Move semantics also enables the creation of move-only types, such as std::unique_ptr, std::future, and std::thread.

• Perfect forwarding makes it possible to write function templates that take arbitrary arguments and forward them to other functions such that the target functions receive exactly the same arguments as were passed to the forwarding functions.

Rvalue references are the glue that ties these two rather disparate features together. They’re the underlying language mechanism that makes both move semantics and perfect forwarding possible.

The more experience you have with these features, the more you realize that your initial impression was based on only the metaphorical tip of the proverbial iceberg. The world of move semantics, perfect forwarding, and rvalue references is more nuanced than it appears. std::move doesn’t move anything, for example, and perfect forwarding is imperfect. Move operations aren’t always cheaper than copying; when they are, they’re not always as cheap ...