One common C++ coding practice is to prefer non-member non-friend functions to member functions. This is a great way to increase encapsulation and keep class interfaces as minimal as possible. When you type a dot . or an arrow -> after an expression, free functions that accept the expression as the first parameter will be suggested in the completion list after member functions:
You can clear this checkbox if you do not want to have free functions in the completion suggestions.
Show import items in basic completion
When a symbol that you want to use is not imported in the current file with the corresponding #include, but it is defined in the standard libraries or elsewhere in your solution, you can press Control+Alt+Space to quickly find this symbol in completion suggestions: You can use this option to include import symbol suggestions into the suggestion lists of automatic and basic completion. Note that this comes with a performance hit (since each time when ReSharper builds a completion list, it has to look at all the symbols in the solution to match them), so it is disabled by default.
Behavior of import items
Configure the import completion behavior to always insert an include directive or show a menu with an option to add a forward declaration.