Policy-Based Design

On my previous post about C++ Policies, a policy-based design was used for comparators and loggers, which are stateless function objects (they only call a single function). These types are empty, but as members they still occupy space because C++ guarantees distinct addresses for distinct subobjects. With C++20’s [[no_unique_address]] or Empty Base Optimization (EBO), programmers can make these truly zero-cost, which is huge for small, hot objects like iterators or adapters.1 ...

I’ve been reading about templates a lot lately and kept seeing the word policy. Which made me realized I wanted a single post that answers the question: What is a policy? Why do C++ sources keep mentioning them? How do compile-time policies (templates) compare to runtime strategies (virtuals/std::function)? And when should which be used? ...