[webkit-dev] How we enable template functions

[Mark Lam]

One application of enable_if I’ve needed in the past is where I want specialization of a template function with the same argument signatures, but returning a different type.  The only way I know to make that happen is to use enable_if in the return type, e.g.

    std::enable_if<std::is_integral<T>, T>::type doStuff() { }
    std::enable_if<std::is_double<T>, T>::type doStuff() { }

This works around the problem of “duplicate function definitions” which arises if the enable_if is not in the function signature itself.  So, I’m not sure your ENABLE_TEMPLATE_IF macro will give me a solution for this.

Mark


> On Aug 22, 2017, at 11:14 PM, Keith Miller <keith_miller at apple.com> wrote:
> 
>> On Aug 22, 2017, at 9:17 PM, JF Bastien <jfb at chromium.org <mailto:jfb at chromium.org>> wrote:
>> 
>> I'd suggest considering what it'll look like when we're migrating to concepts in C++20.
>> 
>> Here's an example for our bitwise_cast:
>> https://github.com/jfbastien/bit_cast/blob/master/bit_cast.h#L10 <https://github.com/jfbastien/bit_cast/blob/master/bit_cast.h#L10>
>> 
>> Notice the 3 ways to enable. There's also the option of using enable_if on the return value, or as a defaulted function parameter, but I'm not a huge fan of either.
> 
> I think the concepts approach is the cleanest. I’d avoid the macro if we go that way.
> 
> But C++20 is a long way away and I only expect this problem to get worse over time. So I’d rather find a nearer term solution.
> 
>> On Aug 22, 2017, at 9:13 PM, Chris Dumez <cdumez at apple.com <mailto:cdumez at apple.com>> wrote:
>> 
>> I personally prefer std::enable_if<>. For e.g.
>> 
>> template<typename T, class = typename std::enable_if<std::is_same<T, int>::value>
>> Class Foo { }
> 
> I just find this much harder to parse since I now have to:
> 
> 1) recognize that the last class is not a actually polymorphic parameter
> 2) figure out exactly what the condition is given that it’s hidden inside an enable_if*
> 
> The plus side of using a static_assert based approach is that it doesn’t impact the readability of function/class signature at a high level since it’s nested inside the body. It’s also not hidden particularly hidden since I would expect it to be the first line of the body
> 
> Another downside of enable_if as a default template parameter is that someone could make a mistake and pass an extra template value, e.g. Foo<float, int>, and it might pick the wrong template parameter. This isn’t super likely but it’s still a hazard.
> 
> Admittedly, we could make a macro like (totes not stolen from JF’s GitHub):
> 
> #define ENABLE_TEMPLATE_IF(condition) typename = typename std::enable_if<condition>::type
> 
> and implement Foo as:
> 
> template<typename T, ENABLE_TEMPLATE_IF(std::is_same<T, int>::value)>
> class Foo { };
> 
> I think this approach is pretty good, although, I think I care about the enable_if condition rarely enough that I’d rather not see it in the signature. Most of the time the code will look like:
> 
> template<typename T, ENABLE_TEMPLATE_IF(std::is_same<T, int>::value)>
> class Foo {...};
> 
> template<typename T, ENABLE_TEMPLATE_IF(std::is_same<T, float>::value)>
> class Foo {...};
> 
> template<typename T, ENABLE_TEMPLATE_IF(std::is_same<T, double>::value)>
> class Foo {...};
> 
> So when I know I want to use a Foo but I forgot the signature I now need to look mentally skip the enable_if macro, which I’d rather avoid.
> 
>> 
>> I don’t like that something inside the body of a class / function would cause a template to be enabled or not.
> 
> I believe there are cases where this already basically already happens e.g. bitwise_cast. Although, I think those cases could be fixed with a more standard approach.
> 
> Cheers,
> Keith
> 
>> 
>> --
>>  Chris Dumez
>> 
>> 
>> 
>> 
>>> On Aug 22, 2017, at 8:34 PM, Keith Miller <keith_miller at apple.com <mailto:keith_miller at apple.com>> wrote:
>>> 
>>> Hello fellow WebKittens,
>>> 
>>> I’ve noticed over time that we don’t have standard way that we enable versions of template functions/classes (flasses?). For the most part it seems that people use std::enable_if, although, it seems like it is attached to every possible place in the function/class.
>>> 
>>> I propose that we choose a standard way to conditionally enable a template.
>>> 
>>> There are a ton of options; my personal favorite is to add the following macro:
>>> 
>>> #define ENABLE_TEMPLATE_IF(condition) static_assert(condition, “template disabled”)
>>> 
>>> Then have every function do:
>>> 
>>> template<typename T>
>>> void foo(…)
>>> {
>>>    ENABLE_TEMPLATE_IF(std::is_same<T, int>::value);
>>>    …
>>> }
>>> 
>>> And classes:
>>> 
>>> template<typename T>
>>> class Foo {
>>>    ENABLE_TEMPLATE_IF(std::is_same<T, int>::value);
>>> };
>>> 
>>> I like this proposal because it doesn’t obstruct the signature/declaration of the function/class but it’s still obvious when the class is enabled. Obviously, I think we should require that this macro is the first line of the function or class for visibility. Does anyone else have thoughts or ideas?
>>> 
>>> Cheers,
>>> Keith
>>> 
>>> P.S. in case you are wondering why this macro works (ugh C++), it’s because if there is any compile time error in a template it cannot be selected as the final candidate. In my examples, if you provided a type other than int foo/Foo could not be selected because the static_assert condition would be false, which is a compile error.
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>>> https://lists.webkit.org/mailman/listinfo/webkit-dev <https://lists.webkit.org/mailman/listinfo/webkit-dev>
>> 
> 
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