Programming Rust, 2nd Edition

Chapter 4. Ownership and Moves

When it comes to managing memory, there are two characteristics we’d like from our programing languages:

• We ʼ d like memory to be freed promptly, at a time of our choosing. This gives us control over the program’s memory consumption.

• We never want to use a pointer to an object after it’s been freed. This would be undefined behavior, leading to crashes and security holes.

But these seem to be mutually exclusive: freeing a value while pointers exist to it necessarily leaves those pointers dangling. Almost all major programming languages fall into one of two camps, depending on which of the two qualities they give up on:

• The “Safety First” camp uses garbage collection to manage memory, automatically freeing objects when all reachable pointers to them are gone. This eliminates dangling pointers by simply keeping the objects around until there are no pointers to them left to dangle. Almost all modern languages fall in this camp, from Python, JavaScript, and Ruby to Java, C#, and Haskell.

  But relying on garbage collection means relinquishing control over exactly when objects get freed to the collector. In general, garbage collectors are surprising beasts, and understanding why memory wasn’t freed when you expected can be a challenge.

• The “Control First” camp leaves you in charge of freeing memory. Your program’s memory consumption is entirely in your hands, but avoiding dangling pointers also becomes entirely your concern. C and C++ are the only ...