Const Correctness
When programming in C++, applying the concept of 'const correctness' can make your code easier to read and understand, as well as more robust and less error-prone. But it's a difficult subject to understand.
This series of 6 articles discusses all aspects of C++ const correctness quite thoroughly, with lots of example code. It progresses step-by-step, starting with simple examples and building in complexity with each article.
Definitely worth a read if you work on a large C++ project.
C++ Const Correctness