Re: [Boost-users] [log] Unable to use own severity struct encapsulated in namespace

Subject: Re: [Boost-users] [log] Unable to use own severity struct encapsulated in namespace
From: Richard Hodges (hodges.r_at_[hidden])
Date: 2018-04-06 08:54:27

On Fri, 2018-04-06 at 09:39 +0200, Sebastian Messerschmidt wrote:
> Hi Richard,
> 
> Thank you for the insight and clarifications. The problem with this
> is 
> certainly, that as an user you're left unaware, that the compiler
> didn't 
> pick your specialized overload, since enums are still sort of
> glorified 
> integers when it comes to types.
> 

Yes, it's one of those "awesome but problematic for beginners" areas of
c++.

ADL allows us to write code in terms of semantics rather than
specifics, which is the awesome part. 

But the problem as you say, is that it takes experience to realise
where which overload will be selected - particularly with free
functions for which there is an overload for more than one class in an
object's public class hierarchy. 

Some discipline (and documentation) is often required.

The good news is that if there is ambiguity, the compiler will demand
programmer intervention, so at least the overloads selection is
predictable, provided you know about the existence of all overloads.

R

> Cheers
> Sebastain >
> > 
> > On 4 April 2018 at 11:24, Sebastian Messerschmidt via Boost-users 
> > <boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>>
> > wrote:
> > 
> >     I've solved the issue:
> > 
> >     In case anyone is interested: You need to put the ostream&
> >     operator<< into the namespace of the severity-enum to make it
> > work.
> > 
> > 
> > This is called ADL - Argument Dependent Lookup.
> > 
> > When c++ encounters a namespace-unqualified function call, it will 
> > consider the current namespace, the global namespace, and the
> > namespaces 
> > enclosing the _/type/_ of each of the function's arguments.
> > 
> > For this reason, it's good practice to declare all free functions 
> > (including operators) that operate on user defined types in the 
> > namespace of those types.
> > 
> > 
> > 
> >     Cheers
> >     Sebastian > Hi folks,
> > 
> > 
> >         When taking this example [1] and modifying the enum
> >         'severity_level' like this:
> > 
> >         namespace x {
> >               struct test
> >               {
> >                   enum severity_level
> >                   {
> >                       normal,
> >                       notification,
> >                       warning,
> >                       error,
> >                       critical
> >                   };
> >               };
> >         }
> > 
> >         and replacing all references to severity_level with the
> >         appropriate x::test::severity_level the 'operator<<'
> > overload
> >         for the severity_level is not issued. Can anyone help me
> > with
> >         this one?
> > 
> >         I can supply the complete modified example if needed
> > 
> > 
> >         Cheers
> >         Sebastian
> > 
> >         [1]
> >         https://www.boost.org/doc/libs/1_58_0/libs/log/example/doc/
> > tutorial_attributes.cpp
> >         <https://www.boost.org/doc/libs/1_58_0/libs/log/example/doc
> > /tutorial_attributes.cpp>
> > 
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