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April 10, 2017 Forwarding calls to objects of another type | ||||
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I have a couple of questions related to the following code: https://gist.github.com/Jaffe-/b027287a884fc2e173a65601ec242676 1) This is a very simplified example, but what I'm trying to do here is to call `foo` on each object in `Container.ss` contains when `foo` is called, and likewise for `bar`. To do this without having to repeat the foreach loop in every member function, I made the `call_for_each` template. However, I found no other way to pull this off (without using string mixins) than to also add the `call` template in the S struct. At first I thought it would be possible to write `s.func(args)` in `call_for_each`, but that will try to look up func as a member in the struct. Is there a more common / idiomatic way of doing this? In C++ this would be solved by using a member function pointer as a template argument, but as far as I understand D uses delegates (which are already bound to an instance) instead? 2) This is about the reduce templates. As I've commented, I can't use a template lambda with reduce, but I can use a lambda taking ints as arguments. Why is this? The error message I get when using the template lambda is: "template instance reduce!((a, b) => a + b) cannot use local '__lambda1' as parameter to non-global template reduce(alias fun)()" |
April 10, 2017 Re: Forwarding calls to objects of another type | ||||
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Posted in reply to Johan Fjeldtvedt | On Monday, 10 April 2017 at 21:04:10 UTC, Johan Fjeldtvedt wrote: > I have a couple of questions related to the following code: https://gist.github.com/Jaffe-/b027287a884fc2e173a65601ec242676 > > 1) This is a very simplified example, but what I'm trying to do here is to call `foo` on each object in `Container.ss` contains when `foo` is called, and likewise for `bar`. To do this without having to repeat the foreach loop in every member function, I made the `call_for_each` template. However, I found no other way to pull this off (without using string mixins) than to also add the `call` template in the S struct. At first I thought it would be possible to write `s.func(args)` in `call_for_each`, but that will try to look up func as a member in the struct. > > Is there a more common / idiomatic way of doing this? In C++ this would be solved by using a member function pointer as a template argument, but as far as I understand D uses delegates (which are already bound to an instance) instead? One way: struct S { int x[]; void foo(int x) { this.x ~= x; } void bar() { writeln("Contains: ", x); } auto call(alias func, T...)(T args) { return func(args); } auto reduce(alias func)() { return x.reduce!(func); } } class Container { S[10] ss; void dispatch(string func, T...)(T args) { foreach(ref s; ss) { __traits(getMember, s, func)(args); } } auto reduce(alias func)() { return ss[] .map!(t => t.reduce!func) .reduce!(func); } } void main() { auto test = new Container(); test.dispatch!"foo"(10); test.dispatch!"foo"(20); test.dispatch!"foo"(30); test.dispatch!"bar"(); //auto x = test.reduce!((a, b) => a + b); auto x = test.reduce!((int a, int b) => a + b); // Compiles writeln(x); } Another way would be to take the delegate of one instance and to patch the pointer to the instance for each instance (because the member of a delegate can be changed: .ptr -> instance, .funcptr -> static address of the function). But it's less clean. > 2) This is about the reduce templates. As I've commented, I can't use a template lambda with reduce, but I can use a lambda taking ints as arguments. Why is this? The error message I get when using the template lambda is: > > "template instance reduce!((a, b) => a + b) cannot use local '__lambda1' as parameter to non-global template reduce(alias fun)()" No idea for this. |
April 11, 2017 Re: Forwarding calls to objects of another type | ||||
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Posted in reply to Basile B. | On Monday, 10 April 2017 at 21:27:34 UTC, Basile B. wrote: >> 2) This is about the reduce templates. As I've commented, I can't use a template lambda with reduce, but I can use a lambda taking ints as arguments. Why is this? The error message I get when using the template lambda is: >> >> "template instance reduce!((a, b) => a + b) cannot use local '__lambda1' as parameter to non-global template reduce(alias fun)()" > > No idea for this. The use of the global identity template will fix this: see https://blog.thecybershadow.net/2015/04/28/the-amazing-template-that-does-nothing/ |
April 19, 2017 Re: Forwarding calls to objects of another type | ||||
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Posted in reply to Basile B. | On Monday, 10 April 2017 at 21:27:34 UTC, Basile B. wrote: > On Monday, 10 April 2017 at 21:04:10 UTC, Johan Fjeldtvedt wrote: >> [...] > > One way: > > [...] Thanks for the reply. The traits way of doing it seems to be what I want. :) >> [...] > > [...] |
April 19, 2017 Re: Forwarding calls to objects of another type | ||||
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Posted in reply to Nicholas Wilson | On Tuesday, 11 April 2017 at 02:01:19 UTC, Nicholas Wilson wrote:
> On Monday, 10 April 2017 at 21:27:34 UTC, Basile B. wrote:
>>> 2) This is about the reduce templates. As I've commented, I can't use a template lambda with reduce, but I can use a lambda taking ints as arguments. Why is this? The error message I get when using the template lambda is:
>>>
>>> "template instance reduce!((a, b) => a + b) cannot use local '__lambda1' as parameter to non-global template reduce(alias fun)()"
>>
>> No idea for this.
>
> The use of the global identity template will fix this:
>
> see https://blog.thecybershadow.net/2015/04/28/the-amazing-template-that-does-nothing/
Thanks, that did work. I think I understand the point about UFCS lookup rules, but it still seems strange (it was at least surprising) that I couldn't use the template (a, b) => a + b in place of (int a, int b) => a + b.
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