C++ System Programming Cookbook

How it works...

The first example with auto shows that the type is deduced, at compile time, from the right-hand parameter. auto is used in simple cases.

decltype() deduces the type of expression. In the example, it defines the y variable so that it's the same type as a. As you can imagine, this would not be possible with auto. Why? This is pretty simple: decltype() tells the compiler to define a variable of a specific type; in the first example, y is a variable with the same type as a. With auto, the type is deduced automatically.

We should use auto and decltype() anytime we don't have to explicitly specify the type of a variable; for example, when we need a double type (and not a float). It's worth mentioning that both auto and decltype() ...