Calling template member function?
Steven Schveighoffer
schveiguy at gmail.com
Tue Apr 19 16:38:42 UTC 2022
On 4/19/22 11:46 AM, Paul Backus wrote:
> On Tuesday, 19 April 2022 at 13:36:26 UTC, Andrey Zherikov wrote:
>> I want to migrate my library API from standalone function that takes
>> delegate as argument to a template member function that takes delegate
>> as a template parameter but compiler errors out. Here is code example:
>> ```d
>> import std.stdio;
>>
>> template T(P)
>> {
>> static void f_new(alias func)()
>> {
>> func();
>> }
>> }
>>
>> void f(FUNC)(FUNC func)
>> {
>> T!int.f_new!(() => func());
>> }
>>
>> void main()
>> {
>> f(function () { __LINE__.writeln; });
>> }
>> ```
>>
>> Compiler error:
>> ```
>> onlineapp.d(7): Error: `static` function `onlineapp.f!(void function()
>> @safe).f.f_new!(delegate () @safe
>> {
>> (*func)();
>> return ;
>> }
>> ).f_new` cannot access delegate `__lambda2` in frame of function
>> `onlineapp.f!(void function() @safe).f`
>> onlineapp.d(13): `__lambda2` declared here
>> onlineapp.d(13): Error: template instance `onlineapp.f!(void
>> function() @safe).f.f_new!(delegate () @safe
>> {
>> (*func)();
>> return ;
>> }
>> )` error instantiating
>> onlineapp.d(18): instantiated from here: `f!(void function()
>> @safe)`
>> ```
>>
>> What confuses me a lot is that if I remove `template T` then
>> everything works perfectly:
>
> The message is different, but I think this error is probably related to
> the "dual context" issue:
>
> https://issues.dlang.org/show_bug.cgi?id=5710
>
> Basically, `f_new!(() => func())` has two scopes that it "wants" to be
> nested in: `T!int` and `f!(void function() @safe)`. When you remove
> `template T`, there's only one scope, so it's not a problem.
No, there is no context pointer necessary for a template instantiation,
without one being artificially introduced via an alias parameter.
`T!int` is essentially just a namespace, especially since the `P`
parameter isn't even used.
>
> If you remove `static` from `f_new`, you get an error message talking
> about this explicitly:
Interesting that `static` does anything there, since it's a no-op.
-Steve
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