[webkit-dev] How we enable template functions

[Chris Dumez]

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 don’t like that something inside the body of a class / function would cause a template to be enabled or not.

--
 Chris Dumez




> On Aug 22, 2017, at 8:34 PM, Keith Miller <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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