Goodness in programming languages, part 4 - Ownership & Memory

Posts in this series: Syntax, Deployment, Metaprogramming, Ownership

There is often a trade-off between programming language features and how fast (and predictably) the programs run. From web sites that serve millions of visitors to programs running on small devices we need to be able to make our programs run quickly.

One trade-off that is made in many modern programming languages (including Python, Ruby, C#, Java and JVM-based languages) is that the system owns all the memory. This avoids the need for the programmer to think about how long pieces of memory need to live, but it means a lot of memory can hang around a lot longer than it really needs to. In addition, it can mean the CPU has to jump around to lots of different memory locations to find pieces of dynamically-allocated memory in different locations. Where this jumping around causes caches to be invalidated that can really slow things down.

While these garbage collection-based languages have been evolving, C++ has been developing along a different track. C++ allows the programmer to allocate and free up memory manually (as in C), but over time the community of C++ programmers has been developing a new way of thinking about memory, and developing tools in the C++ language to make it easier to work in this way.

Modern C++ code rarely or never uses "delete" or "free" to deallocate memory, but instead defines clearly which object owns each other object. When the owning object is no longer needed, everything it owns can be deleted, immediately freeing their memory. The top-level objects are owned by the current scope, so when the function or block of code we are in ends, the system knows these objects and the ones they own can be deleted. Objects that last for the whole life of the program are owned by the scope of the main function or equivalent.

One advantage of explicit ownership is that the right thing happens automatically when something unexpected happens (e.g. an exception is thrown, or we return early from a function). Because the objects are owned by a scope, as soon as we exit that scope they are automatically deleted, and no memory is "leaked".

Because ownership is explicit, we can often group owned objects in memory immediately next to the objects that own them. This means we jump around to different memory locations less often, and we have to do less work to find and delete regions of memory. This makes our programs faster.

Here are some things I like:

• Modern C++'s clarity about who owns what. By expressing ownership explicitly we make clear our intentions, and avoid memory leaks.
• Modern C++'s fast and cache-friendly memory handling. Allocating memory for several objects together reduces time spent looking for space, and means caches are more likely to be used.

In my experience, the most frequent performance problems I have had to solve have really been memory problems. Explicit ownership can reduce unnecessary memory management overhead by taking back the work from the system (the garbage collector) and allowing programmers to be explicit about who owns what.