Re: [Boost-users] [boost] Versioned namespaces (was Rebooting Boost: Call for Lightning Talks @ CppCon)

Subject: Re: [Boost-users] [boost] Versioned namespaces (was Rebooting Boost: Call for Lightning Talks @ CppCon)
From: Michael (mwpowellhtx_at_[hidden])
Date: 2016-05-22 09:36:44

On May 22, 2016 9:01:41 AM EDT, "Vicente J. Botet Escriba" <vicente.botet_at_[hidden]> wrote:
>Le 22/05/2016 à 13:18, Niall Douglas a écrit :
>>
>> Here is an example of such a lightning talk:
>>
>>
>> 1. Current design pattern:
>>
>> Library v1.00:
>> namespace boost { namespace lib { void foo(size_t, char); } }
>>
>> User code:
>> boost::lib::foo(100, 'n');
>>
>>
>> 2. Problem:
>>
>> Library v1.01:
>> namespace boost { namespace lib { void foo(char, size_t); } }
>>
>> User code:
>> boost::lib::foo(100, 'n');  // oh oh!

In my opinion, this is the wrong question you should be asking.

I don't know your organizational constraints or how you enforce version of third party libraries, but that very activity is the better question you should be asking.

HTH

>> 3. Alternative:
>>
>> Library v1.00:
>> namespace boost { namespace lib { inline namespace v100 { void
>> foo(size_t, char); } } }
>> #define BOOST_LIB_NAMESPACE_V100 boost::lib::v100
>>
>> Library v1.01:
>> namespace boost { namespace lib { inline namespace v101 { void
>> foo(char, size_t); } } }
>> #define BOOST_LIB_NAMESPACE_V101 boost::lib::v101
>>
>> User code:
>> // Mount v100 of Boost.Lib into "lib"
>> namespace lib=BOOST_LIB_NAMESPACE_V100;
>> lib::foo(100, 'n');  // ABI stable! No ODR violation possible!
>I believed that minor version should be ABI compatible. I don't see the
>
>need to use namespaces for minor versions. When do you need to version
>them?

I didn't know what the heck y'all were talking about with so-called "ABI compatibility", but now I do, or at least I have a better idea. But now I do; it just sounds to me more like a non-solution that cops out of a non-problem.

In my opinion, this should never be the question. What ever happened to maintaining backwards compatibility, managing breaking changes, etc. Even at the risk of offending someone.

>Why do you need macros here? What is wrong with
>
>namespace lib=boost::lib::v100;
>
>?
>>
>> 4. Problems fixed:
>>
>> a) Mixing incompatible versions of individual Boost libraries in the
>> same process (or even translation unit) is now safe.
>As far as all applications are using a specific version.
>I don't think this is a good goal. How different versions interact?
>Wouldn't this increase the current interoperability problem between 
>boost and the standard library, but now between several boost versions?
>>
>> b) Libraries can now depend on other libraries without worrying about
>> breaking code which depends on them which uses a different version of
>> the same dependency. This greatly helps reduce change ripple effects.
>Agreed.
>>
>> c) Forward compatibility or alternative implementation shims are now
>> very easy to implement e.g. library users can externally configure
>> either Boost.Filesystem or std::experimental::filesystem for your
>> library to use, your code simply refers to _some_ Filesystem TS
>> implementation e.g.
>>
>> namespace filesystem = FILESYSTEM_TS_IMPLEMENTATION;
>> filesystem::path mypath;
>>
>Hmm, if Boost.Filesystem is a conforming implementation of the TS 
>Filesystem, it should provide a  a <filesystem> file included in the 
>std::experimental::filesystem namespace and then the user wouldn't need
>
>to do anything at the source level, but at the configuration level.
>
>I would add a section of what problem could be introduced ;-)
>Should the versions duplicate the whole code? If not, how this 
>duplication is avoided?
>How the user can partially specialize a class? inside the versioned or 
>not versioned namespace? If we want to allow several version on a 
>process, it should be the versioned one.
>
>What is the advantage of inlining the version namespaces if the user 
>cannot use it directly?
>
>Best,
>Vicente
>
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