Re: [boost] [GSoC] [Boost.Hana] Formal review request

Subject: Re: [boost] [GSoC] [Boost.Hana] Formal review request
From: Robert Ramey (ramey_at_[hidden])
Date: 2014-08-03 11:48:56

Roland Bock-2 wrote
> On 2014-08-02 23:57, pfultz2 wrote:
>>> I would just like every library invoke a compile time assert whenever
>>> > in instantiate a template with type arguments which won't work.
>>> > Boost Concept Check can do this.  In fact, maybe BCC is over kill.
>>> > I seriously considered recommending making concept checking classes
>>> with
>>> > just static_assert.  But in the end I opted for recommending BCC. 
>> Yes, of course, static assertions can help improve error messages and its
>> better than nothing, but it is not the same as template constraints.
> True, but static_assert covers the following cases at least and is
> really simple:
> 
> a) You have one implementation (class or function) and arguments are
> either valid or not:
> 
> template<...>
> struct X
> {
>     static_assert(check1, "failed check1");
>     ...
> };
> 
> b) You have one or more full or partial class specializations for
> specific classes/templates/values:
> 
> Basically as above, but if you want to prohibited the default case or
> certain specializations, here's a nice, minimalistic helper to do so [1]:
> 
> namespace detail
> {
>   template
> <typename... T>
>     struct wrong
>     {
>       using type = std::false_type;
>     };
> }
> template
> <typename... T>
>   using wrong_t = typename detail::wrong
> <T...>
> ::type;
> 
> It can be used to defer static_assert until a template is instantiated
> which is an error since it is prohibited:
>  
> 
> // disabled default case, slightly shortened from [2] template
> <typename
> Context, typename T>
> struct serializer_t
> {
>   static_assert(wrong_t
> <serializer_t>
> ::value, 
> "missing serializer specialization");
> };
> 
> // Any number of valid specializations like this [3]
> template
> <typename Context, typename... Args>
> struct serializer_t&lt;Context, column_t&lt;Args...&gt;>
> {
>   // You could have static asserts here, too
> };
> 
> // Or you can switch off certain cases [4]:
> template&lt;typename Lhs, typename Rhs, typename On&gt;
> struct serializer_t&lt;sqlite3::serializer_t, join_t&lt;outer_join_t, Lhs,
> Rhs, On&gt;>
> {
>    static_assert(::sqlpp::wrong_t
> <serializer_t>
> ::value, "No support for
> outer join");
> };
> 
> 
> 
> static_assert is not sufficient for selecting overloads or if you have
> several class specializations and need to select the correct one based
> on concepts like ForwardIterator or so. In those cases, I would use
> enable_if or similar (and wait for Concepts Lite).
> 
> In the sqlpp11 code, which enforces type constraints in pretty much
> every template, enable_if is required in only about 7% of the templates.
> Most of them are implementations of the constraint-checks.
> 
> Regards,
> 
> Roland
> 
> 
> [1]: https://github.com/rbock/sqlpp11/blob/master/include/sqlpp11/wrong.h
> [2]:
> https://github.com/rbock/sqlpp11/blob/develop/include/sqlpp11/serializer.h
> [3]: https://github.com/rbock/sqlpp11/blob/master/include/sqlpp11/column.h
> [4]:
> https://github.com/rbock/sqlpp11-connector-sqlite3/blob/develop/include/sqlpp11/sqlite3/serializer.h
> 
> _______________________________________________
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I see using static_assert as a real practical method for many users.
But just saying one can use static_assert to implement type constraints is
not quite the 
same as showing users how to do it. I would encourage you go to enhance
http://rrsd.com/blincubator.com/advice_concepts/
with this information.  You an either post a comment or send it directly to
me.

My real goal is to convince the C++ community that any library which
includes templates as part of the user interface must have explicit type
constraints
in order to be considered for formal review.  I want to see type constraints
be understood and appreciated by the C++ user community in general rather
than
a small group who spend years disputing how to implement the idea.

Let's start making things better right now!

Robert Ramey

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