using-declaration or typedef for iterator tags?

Question

I'm creating a class representation of a mathematical tuple (or simply, an ordered set of numbers). Being a list-like object that could benefit from an iterator representation, I have decided to give it one. By following this tutorial, I have found little difficulty in creating the iterator class.

What I do not understand, however, is why the author used alias-declarations instead of typedefs when declaring the iterator's tags. I have seen other people doing this as well, such as here. I would prefer to use typedefs as it makes things clearer when creating/reading member functions, but I'm unaware if there are any downsides to doing this.

Edit: Changed using-declarations to alias-declarations, as pointed out by Sebastian Redl

Answer 1

In C++, using and typedef are mostly equivalent and both declare a type alias. But they have reverse order of arguments:

typedef original new_name;
using new_name = original;

The typedef can get very difficult to read for more complex types. For example, the meaning of typedef unsigned uint might not be immediately apparent. C's notation for function pointers is famously inscrutable: typedef void (*FuncType)(int a, int b). This syntactic mess also means you can't use a typedef template. Unfortunately, typedef also has very subtle interactions with linkage (which I don't understand and thus can't explain here, sorry).

The type alias with using is generally easier to read and is just a type alias, without other subtle effects. It composes well with templates. It was introduced as part of the big language modernization in C++11. Modern C++ greatly prefers this form, see also T.43 in the C++ Core Guidelines. The using keyword is also used for other constructs that bring something into scope, e.g. namespace aliases or using-declarations for unqualified lookup (argument-dependent lookup).

The syntax used for declaring type aliases does not interact or interfere with member functions.

So you can continue to use typedef without any ill effects. However, the C++ community would thank you if you don't use that C-ism and use the more easier to read using keyword.

Answer 2

These aren't using-declarations. They are alias-declarations. Aside from the different syntax, they are 100% semantically identical to typedefs.

I find them a lot more readable, though, and I imagine the author of the tutorial has the same view, which would be reason enough to use them.