Issue 5694: Dot_configuration maintenance

https://sourceforge.net/p/testlilyissues/issues/5694/

1: iterate properly in reverse
The previous code would have malfunctioned for an empty container.

2: swap instead of copy

3: reduce repetition using a lambda function

4: use range-for loops

5: Dot_configuration "has a" instead of "is a" std::map
    
It is considered poor form to create subclasses of standard containers
because they lack virtual destructors.  Inheriting privately from
std::map avoids potential problems by preventing Dot_configuration from
being treated as a std::map from the outside.  (I didn't find any actual
problems in this case.)
    
Limiting the Dot_configuration interface to just the map methods that
are actually used makes it easier to understand how Dot_configuration is
used without searching the entire code.

[Dan Eble, 2020-01-25 16:13:02]

swap instead of copy

[Dan Eble, 2020-01-25 16:14:01]

reduce repetition using a lambda function

[Dan Eble, 2020-01-25 16:14:24]

use range-for loops

[Dan Eble, 2020-01-25 16:14:53]

Dot_configuration "has a" instead of "is a" std::map

[hanwenn, 2020-01-25 18:24:32]

https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
File lily/dot-configuration.cc (right):

https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
lily/dot-configuration.cc:69: auto process_entry = [d, k, &new_cfg,
&offset](const value_type &ent)
what does this syntax do?

[Dan Eble, 2020-01-25 18:37:23]

On 2020/01/25 18:24:32, hanwenn wrote:
>
https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
> File lily/dot-configuration.cc (right):
> 
>
https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
> lily/dot-configuration.cc:69: auto process_entry = [d, k, &new_cfg,
> &offset](const value_type &ent)
> what does this syntax do?

https://en.cppreference.com/w/cpp/language/lambda

It defines a function named "process_entry" taking one argument (const
value_type &).  The body of the function can access variables d and k by value,
and variables new_cfg and offset by reference.  I think "this" might also be
captured implicitly in this context, but I'd have to go to the documentation to
refresh my memory.

An alternative to using a lambda would be to move this stuff to a private member
function, or maybe a static function in this file.

[hanwenn, 2020-01-25 18:45:28]

On Sat, Jan 25, 2020 at 7:37 PM <nine.fierce.ballads@gmail.com> wrote:
>
> On 2020/01/25 18:24:32, hanwenn wrote:
> >
>
https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
> > File lily/dot-configuration.cc (right):
> >
> >
>
https://codereview.appspot.com/577380046/diff/553430046/lily/dot-configuratio...
> > lily/dot-configuration.cc:69: auto process_entry = [d, k, &new_cfg,
> > &offset](const value_type &ent)
> > what does this syntax do?
>
> https://en.cppreference.com/w/cpp/language/lambda
>
> It defines a function named "process_entry" taking one argument (const
> value_type &).  The body of the function can access variables d and k by
> value, and variables new_cfg and offset by reference.  I think "this"
> might also be captured implicitly in this context, but I'd have to go to
> the documentation to refresh my memory.
>
> An alternative to using a lambda would be to move this stuff to a
> private member function, or maybe a static function in this file.

Thanks for the explanation!

-- 
Han-Wen Nienhuys - hanwenn@gmail.com - http://www.xs4all.nl/~hanwen