Here's a nice detailed treatment of auto and decltype by longtime author Thomas Becker.

C++ auto and decltype Explained

by Thomas Becker

From the article:

A while later, sometime in 2012, I noticed that there was another feature, or rather, a pair of features, in C++11 that I had not fully understood, namely, the auto and decltype keywords. With auto and decltype, unlike rvalue references, the problem is not that they are difficult to grasp. On the contrary, the problem is that the idea is deceptively easy, yet there are hidden subtleties that can trip you up.

Let's start with a good look at the auto keyword...