[webkit-dev] How we enable template functions

[JF Bastien]

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

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.

On Tue, 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 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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