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Subject: Re: [boost] [Boost.Breakable] Any interest in a Boost Breakable library?
From: Ilya Bobir (ilya.bobir_at_[hidden])
Date: 2009-09-09 11:58:21

Stewart, Robert wrote:
> Ilya Bobir wrote:
>> Here is a real life sample I was able to find using Google
>> Code search:
>> We are trying to load a cursor. There are 3 different points were it
>> can fail. And we want to return a default cursor in case of
>> any failure.
> That's a horrible example. The loop is used to jump to cleanup code that should be handled using RAII (a sentry would work nicely).

Your answer reminded me talks on why closures should not be added to
Java. There is a point that as Java has anonymous classes there really
is no use for closures as one can emulate a closure with an anonymous class.

While I do think that RAII is a very good approach it just does not do
well in case you have to deal with a C style API. Windows API is mostly
C style. As well as the old DirectX API.

If all APIs were C++ and were codded to support RAII and exceptions
programming in C++ would be a lot easier but it is not the case. Some
of C++ programmers still have to deal with C APIs and wrapping
everything to support RAII is not always the most cost effective approach.

>> Another context were this kind of logic may appear are
>> parsers. Here is
>> another real life sample:
>> I do not think that "cutting out" the "do { ... } while
>> (false);" part
>> into a separate function would add readability.
> It would work nicely to create a separate function. The subfunction can return early with a flag that indicates failure and the main function can assign to status and return 0.0. Which is clearer would depend upon the reader, I suppose.

This way you may end up with a lot of functions that are named very
similarly and call each other doing only very small pieces of work.
While generally it seems to be a good thing when your functions are
short when they became too short, because you are blindly adhering to
some coding practice, the code looses readability.

I think this is a good example were creating a separate subfunction will
  not make code more readable.

I have seen some prominent examples in the Windows source of a code that
is hard to understand because a simple thing is performed by a dozen of
functions that call each other. It is hard to understand what a
separate function does as the code is very tightly coupled but because
of some coding convention it is split into different functions. I can
not give you a direct link as the Windows source does not seems to be
available online, but here is a stack trace of a Windows Shell call:

shell32.dll!AicpMsgWaitForCompletion() + 0x36
shell32.dll!AicpAsyncFinishCall() + 0x2c
shell32.dll!AicLaunchAdminProcess() + 0x2ee
shell32.dll!_SHCreateProcess() + 0x59d0
shell32.dll!CExecuteApplication::_CreateProcess() + 0xac
shell32.dll!CExecuteApplication::_TryCreateProcess() + 0x2e
shell32.dll!CExecuteApplication::_DoApplication() + 0x3c
shell32.dll!CExecuteApplication::Execute() + 0x33
shell32.dll!CExecuteAssociation::_DoCommand() + 0x5b
shell32.dll!CExecuteAssociation::_TryApplication() + 0x32
shell32.dll!CExecuteAssociation::Execute() + 0x30
shell32.dll!CShellExecute::_ExecuteAssoc() + 0x82
shell32.dll!CShellExecute::_DoExecute() + 0x4c
shell32.dll!CShellExecute::s_ExecuteThreadProc() + 0x25
shlwapi.dll!WrapperThreadProc() + 0x98
kernel32.dll!@BaseThreadInitThunk_at_12() + 0x12
ntdll.dll!__RtlUserThreadStart_at_8() + 0x27

The call is supposed to start a process. As you can see method names
are very similar and, I believe, unless you are familiar with the code
it is not obvious what each function does and unless you have a stack
trace it is not even obvious the order in which the functions are called.

This stack trace is a quote from this article:

> The point is that one *can* always rewrite the code a different way. Perhaps the "breakable" approach makes certain code clearer, but it seems to apply to code that is already not exception safe and very C-like.

Totally agreed. Note that not all the code out there is a proper C++
style and there are billions lines of code already written.

>> Cretan languages support this "breakable" construct natively.
>> OvermindDL1 wrote that D have something similar and it was designed
>> quite recently. I can add that Perl have direct support for
>> this kind
>> of control flow. In Perl it looks like this:
> That other languages do something doesn't mean it is a good idea, nor does its being in Perl mean it shouldn't be in C++. ;)

The point it that the breakable is a control flow construct already
"accepted" by some languages. And as such it can be treated as a
control flow pattern. Not as common as "for" but still a control flow
pattern, not a "bad coding style".

Ilya Bobir

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