I have defined a class that's meant to represent a data series, which has an index and a set of values.  Sometimes the user wants to specify a particular index of custom type, other times they don't care and we want to default to an array of contiguous "int" starting from 0.

I have attempted to create a class where the index type is a parameter, but defaults to int.  I also tried to create two constructors:  one for if the index values are not specified (in which the constructor makes the array of ints); and one where the user passes in a literal of values that match the specified type.

However, it seems that only the first constructor is getting called, even though I am passing in two parameters instead of one.  Why isn't the call matching the second constructor and behaving as intended?


 import std.stdio;

 class MyClass(T, U = int) {

    T[] values;
    U[] index;

    this(T[] values) {
        this.values = values;
        // Default index of contiguous ints
        for (int i; i < values.length; i++) {
            index ~= i;
        }
    }

    this(T[] values, U[] index) {
        this.values = values;
        this.index = index;
    }
}

void main() {
    auto myc1 = new MyClass!(int)([1,2,-3]);

    auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b", "c"]); // Error: cannot append type int to type string[]

}

On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:
> I have defined a class that's meant to represent a data series, which has an index and a set of values.  Sometimes the user wants to specify a particular index of custom type, other times they don't care and we want to default to an array of contiguous "int" starting from 0.
>
> I have attempted to create a class where the index type is a parameter, but defaults to int.  I also tried to create two constructors:  one for if the index values are not specified (in which the constructor makes the array of ints); and one where the user passes in a literal of values that match the specified type.
>
> However, it seems that only the first constructor is getting called, even though I am passing in two parameters instead of one.  Why isn't the call matching the second constructor and behaving as intended?
>
>
>  import std.stdio;
>
>  class MyClass(T, U = int) {
>
>     T[] values;
>     U[] index;
>
>     this(T[] values) {
>         this.values = values;
>         // Default index of contiguous ints
>         for (int i; i < values.length; i++) {
>             index ~= i;
>         }
>     }
>
>     this(T[] values, U[] index) {
>         this.values = values;
>         this.index = index;
>     }
> }
>
> void main() {
>     auto myc1 = new MyClass!(int)([1,2,-3]);
>
>     auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b", "c"]); // Error: cannot append type int to type string[]
>
> }

As the error states:
you are trying to append an int to a string array in the single parameter constructor.

This would work:
´´´
import std.stdio;

 class MyClass(T, U = int) {

    T[] values;
    U[] index;

    this(T[] values) {
        this.values = values;
        // Default index of contiguous ints
        static if(is(U == int))
        {
            for (int i; i < values.length; i++) {
                index ~= i;
            }
        }
    }

    this(T[] values, U[] index) {
        this.values = values;
        this.index = index;
    }
}

void main() {
    auto myc1 = new MyClass!(int)([1,2,-3]);

    auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b", "c"]); // Error: cannot append type int to type string[]
}
´´´

If design matters, I would even to expand the static if above the constructor. So, the single parameter constructor would exist iff is(U == int)

´´´
static if(is(U == int))
    {
        this(T[] values) {
        this.values = values;
        // Default index of contiguous ints

            for (int i; i < values.length; i++) {
                index ~= i;
            }
        }
    }
´´´

On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:
> On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:
>> [...]
>
> As the error states:
> you are trying to append an int to a string array in the single parameter constructor.
>
> [...]

Thanks for the response, Alex!  But it's not clear to me why the first constructor is called at all.  Since I called with two parameters, shouldn't it invoke the second constructor?

On Monday, 28 January 2019 at 19:15:04 UTC, Zak wrote:
> On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:
>> On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:
>>> [...]
>>
>> As the error states:
>> you are trying to append an int to a string array in the single parameter constructor.
>>
>> [...]
>
> Thanks for the response, Alex!  But it's not clear to me why the first constructor is called at all.  Since I called with two parameters, shouldn't it invoke the second constructor?

I think I just realized the answer:  this section of code is not called, it just fails compilation since it's not known that runtime doesn't do something like:

auto myc = new MyClass!(int, string)([1,2,-3]);

which "would" invoke this code block with type string.

On Monday, 28 January 2019 at 19:24:21 UTC, Zak wrote:
> On Monday, 28 January 2019 at 19:15:04 UTC, Zak wrote:
>> On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:
>>> On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:
>>>> [...]
>>>
>>> As the error states:
>>> you are trying to append an int to a string array in the single parameter constructor.
>>>
>>> [...]
>>
>> Thanks for the response, Alex!  But it's not clear to me why the first constructor is called at all.  Since I called with two parameters, shouldn't it invoke the second constructor?
>
> I think I just realized the answer:  this section of code is not called, it just fails compilation since it's not known that runtime doesn't do something like:
>
> auto myc = new MyClass!(int, string)([1,2,-3]);
>
> which "would" invoke this code block with type string.

Yes. :)