auto deduces the type the way a function template parameter would: it strips top-level const and references by default. So auto a = ref declares a as a fresh int copy, regardless of whether the initialiser is const int& or just int. Use auto to escape the verbose iterator types in the standard library; the alternative for std::map<std::string, int>::const_iterator is unreadable. Keep types explicit at API boundaries (function signatures, public class fields) where the type IS the documentation.

Add & after auto (and optionally const) to keep reference semantics. auto& is what you want when iterating over a container and need to either mutate or avoid the copy of a non-trivial type. const auto& is the safest default for read-only views because the compiler will catch accidental mutation. The rule of thumb: prefer const auto& in for loops over containers of strings, vectors, maps, or any user-defined type with non-trivial copy.

decltype(expr) yields the type of the expression without evaluating it, preserving cv-qualifiers and reference-ness. That is what you want when writing perfect-forwarding wrappers or trailing return types: auto wrap(T t) -> decltype(t.size()) { return t.size(); }. decltype(auto) (C++14) is the deducer-form that keeps references, useful when forwarding the return value of an inner call without losing aliasing. Reach for decltype only when ordinary auto loses the information you need; in everyday code, auto is enough.