Prefer `unique_ptr` over `shared_ptr` unless you need to share ownership

Reason

A unique_ptr is conceptually simpler and more predictable (you know when destruction happens) and faster (you don't implicitly maintain a use count).

Example, bad

This needlessly adds and maintains a reference count.

void f()
{
    shared_ptr<Base> base = make_shared<Derived>();
    // use base locally, without copying it -- refcount never exceeds 1
} // destroy base

Example

This is more efficient:

void f()
{
    unique_ptr<Base> base = make_unique<Derived>();
    // use base locally
} // destroy base

Enforcement

(Simple) Warn if a function uses a Shared_pointer with an object allocated within the function, but never returns the Shared_pointer or passes it to a function requiring a Shared_pointer. Suggest using unique_ptr instead.