Re: [Swig-devel] C++11 using statements / type alias

[William S F. <ws...@fu...>]

On 23 June 2016 at 19:12, Lior Goldberg <lio...@gm...> wrote:
> On Sun, Jun 19, 2016 at 5:31 PM William S Fulton
> <ws...@fu...> wrote:
>>
>> On 17 June 2016 at 16:39, Lior Goldberg <lio...@gm...> wrote:
>> > Hi all,
>> >
>> > I noticed that C++11 using statements (type alias) such as:
>> >  * using A = int;
>> >  * template <typename T> using ptr_t = T*;
>> > are not currently supported by SWIG.
>> >
>> > Attached is a patch that adds support for these cases, by treating them
>> > as the equivalent typedef statements.
>> >
>> > Note: an alias template needs to be declared in the swig file as follows:
>> >     %template() ptr_t<int>;
>> >
>> > What do you think?
>> >
>> > (I will be happy to revise the code according to your comments as needed)
>> >
>>
>> That's a nice approach for finishing off an important C++11 feature,
>> thanks! Let's split the patch into two parts.
>>
>> 1) Type alias
>> This is just a typedef in C++11 syntax. Patch looks okay. I think we
>> should add an attribute like SetFlag($$, "typealias") into the typedef
>> node indicating that it originates from a using statement and is a
>> type alias rather than a C style typedef. This info may not be used in
>> the code, but is useful info when debugging and for the -xml option.
>>
>> 2) Alias templates
>> While your patches seem to work, I'm undecided if this is all that is
>> required. The requirement to use the empty template instantiation,
>> %template(), seems an unnecessary burden to place on users. Normally a
>> %template with a name is required to instantiate templates so that the
>> template has a name in the target language, but for alias templates,
>> this is not necessary. Hence I think that ideally, SWIG should not
>> require a user to use %template() to support alias templates. There
>> seem to be two cases:
>>
>> a) alias template resolves to a non-template type, eg:
>>   template<typename T> using ptr_t = T*;
>>   ptr_t<int> pi; // pi is a plain primitive type int
>>
>> b) alias template resolves to a template type, eg:
>>   template<typename T> using Vector = std::vector<T>;
>>   Vector<int> vi;
>>
>>   SWIG usage would then need to be:
>>
>>   %include <std_vector.i>
>>   template<typename T> using Vector = std::vector<T>;
>>   %template() Vector<int>; // This empty template instantiation seems
>> really redundant!
>>   %template(VectorInt) std::vector<int>;
>>   Vector<int> vi;
>>
>>   This is actually very similar to something that is currently a
>> nuisance, but not a very prevalent problem atm, eg sometimes the
>> entire std::vector is not required in the wrappers, but the type
>> deduction of the typedefs in the template are, such as:
>>
>>   %template() std::vector<bool>; // this is needed (empty %template()
>> or a name provided to %template)  otherwise SWIG doesn't recognise the
>> type below is actually a bool
>>   std::vector<bool>::value_type mybool;
>>
>>   As template aliases are becoming very common, I think SWIG ought to
>> improve this sooner rather than later by having some sort of automatic
>> empty template instantiation if a user has not provided a name to
>> %template. I suspect this will not be easy to solve properly though
>> but would be a great enhancement.
>>
>> Taking this forward, can you put together these improvements into
>> github pull requests? Let's have one request for each of these two
>> improvements. They'll need suitable changes in the CPlusPlus11.html
>> documentation including some examples and plenty of test cases added
>> to the test-suite. There are extensive ways that type aliases can be
>> declared, eg in different scopes and different types. I suggest the
>> cpp11_type_aliasing.i and cpp11_template_typedefs.i testcases are
>> expanded and runtime tests added. Most of the C++11 runtime tests are
>> in Java or Python, so runtime tests in both or one of these languages
>> is preferred. Lastly, please try and use the correct whitespace in
>> your patches.
>>
>> William
>
> Hi,
>
> I created a pull request for the first part (type alias).
> Regarding alias templates, I agree that a %template() statement is
> completely redundant. But as you mentioned it is part of a bigger
> feature, and therefore should have its own pull request, in my
> opinion. I will try to look into it in the coming days and meanwhile
> work on the alias templates pull request (without this feature).
>

Lior, thank you very much. I've now merged in the type alias pull
request. I'm sure I speak on behalf of many other C++11 users in
saying I look forward to the 2nd pull request.

William