[webkit-dev] How we enable template functions

[Keith Miller]

> On Aug 22, 2017, at 9:17 PM, JF Bastien <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> 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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> 

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