Other C++ Conventions

We have a list of C++ coding conventions here, but there's other aspects of the C++ source code design which should be standardized. I'm starting this issue to keep track of such things.

• use of _t and _type suffixes for typedefs
• decide whether to use _ as prefix or suffix on protected/private members
• use m_ prefix for members
• insist on out-of-line definitions for functions that are longer than one line
• put out-of-line definitions to be inline'd in .ipp files
• names of polymorphic copy and comparisons
• how to reference types from dependencies
  • some of our repos define a header types.hpp do we want to standardize it?
• directory structures?
• when to use nested namespaces
• where to put traits
• when to start new files

This should be compared with the C++ Core Guidelines for consistency.