Solving Problems With the One Definition Rule

Phil Karlton was right on point when he said the following:

"There are two hard things in computer science: cache invalidation and naming things."

Names are difficult for a few reasons: they have to be precise, simple, short, and expressive at the same time. That makes them meaningful and allows programmers to understand the concepts behind the raw implementation. C++ and many other languages impose one more requirement: many names must be unique.

This is manifested in a few different ways. A programmer is required to follow the ODRThe One Definition Rule (ODR) in C++ states that a program should have only one definition for a given entity within a particular scope to avoid conflicts and ensure consistency.. This says that in the scope of a single translation unit (a single .cpp file), we must define it exactly once, even if we declare the same name (of a variable, function, class type, enumeration, concept, or template) multiple times.

This rule is extended to the scope of an entire program for all variables effectively used in our code and non-inlined functions.

ODR-fail example

Consider the following example. We declare a variable i in a shared.h file: