What does this nested namespace definition do?

Question

I don't even know how to search for that but I tried to test it, still could not understand it. What is the code below means:

boost::asio::io_service::id service::id;

Got it from: MaidSafe-CRUX at GitHub

Answer 1

:: is the scope resolution operator. The name on the left-hand side denotes a scope; it can be either a namespace name or a class name. The name on the right-hand side denotes a member of that scope.

In your case, there seems to be a class maidsafe::crux::detail::service, which contains a static member id of type boost::asio::io_service::id. In many cases, static members need an out-of-class definition, which is being provided by the code you've posted.

It can be parsed as any other C++ declaration: it declares something named service::id of type boost::asio::io_service::id. Let's look at the individual components.

Type:

• boost is a namespace (in global scope)
• asio is a namespace inside boost
• io_service is either a namespace or class inside boost::asio
• id is a type inside boost::asio::io_service

Name:

• service is a class name (probably inside namespace maidsafe::crux::detail)
• id is a static member inside maidsafe::crux::detail::service.

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Addendum

While it does not appear in this question, there's a related syntax you may sometimes encounter in more modern C++ code:

namespace maidsafe::rux::detail
{

boost::asio::io_service::id service::id;

}

This has been introduced in C++17 as a short-hand for opening multiple namespaces on one line. It's exactly equivalent to the code in the question.