Why are aliases for template parameters declared in the standard library at the beginning of the class?

Question

C++ / C#

The question is in the title. For readability?

Answer 1

This is done for several reasons.
First, so that the template parameter can be accessed in other templates. Here, you gave an example with _Сontainerin some iterator. You can imagine that there is some kind of algorithm (function) that takes an iterator and it will need to somehow use the container type (or create a temporary variable, or set the return value type, or somehow check that the container type has some properties).
In this case, the algorithm should receive this one back_insert_iteratorand a dozen other iterators. Therefore, it is impossible to take out the container type into a template parameter and specify it in the iterator type in the function parameter. You look in the source code for this very "_Container" - it will become clear how it is used.
Secondly, sometimes it is done for convenience. For in these templates, the parameters themselves can be hellishly piled up with templates.