I ran in to this bug from 2013 tonight:

https://issues.dlang.org/show_bug.cgi?id=10689

Briefly, templated (and overloaded) constructor syntax is supported:

    this(int MAX_UNPACK = BCF_UN_ALL, T)(T *h, bcf1_t *b)
    if(is(T == VCFHeader) || is(T == bcf_hdr_t))
    {
...
    }
    this(SS)(VCFHeader *vcfhdr, string chrom, int pos, string id, string _ref, string alt, float qual, SS filter, )
    if (isSomeString!SS || is(SS == string[]))
    {
...
    }
    this(int MAX_UNPACK = BCF_UN_ALL)(VCFHeader *vcfhdr, string line)
    {
...
    }


But there is no way (?) to instantiate these with specific template value parameters. (Deduction works just fine)

    auto rr = new VCFRecord!BCF_UN_STR(vw.vcfhdr, "...");

for example, returns the same error it did in 2013:

Error: template instance `VCFRecord!BCF_UN_STR` VCFRecord is not a template declaration, it is a class



Suggestions given in past threads include templating the class, but in this, and other posters' examples, the constructor is overloaded with several ways to construct the object.  Too, I am not sure how templating the class would work with template value parameters (vice type params).

Given that templating the constructor is well-supported syntactically, the suggestion to use a factory function seems inelegant.

Thanks for listening and suggestions.
James

On Sunday, 27 January 2019 at 01:22:09 UTC, James Blachly wrote:
> I ran in to this bug from 2013 tonight:
>
> https://issues.dlang.org/show_bug.cgi?id=10689
>
> Briefly, templated (and overloaded) constructor syntax is supported:
>
>     this(int MAX_UNPACK = BCF_UN_ALL, T)(T *h, bcf1_t *b)
>     if(is(T == VCFHeader) || is(T == bcf_hdr_t))
>     {
> ...
>     }
>     this(SS)(VCFHeader *vcfhdr, string chrom, int pos, string id, string _ref, string alt, float qual, SS filter, )
>     if (isSomeString!SS || is(SS == string[]))
>     {
> ...
>     }
>     this(int MAX_UNPACK = BCF_UN_ALL)(VCFHeader *vcfhdr, string line)
>     {
> ...
>     }
>
>
> But there is no way (?) to instantiate these with specific template value parameters. (Deduction works just fine)
>
>     auto rr = new VCFRecord!BCF_UN_STR(vw.vcfhdr, "...");
>
> for example, returns the same error it did in 2013:
>
> Error: template instance `VCFRecord!BCF_UN_STR` VCFRecord is not a template declaration, it is a class
>
>
>
> Suggestions given in past threads include templating the class, but in this, and other posters' examples, the constructor is overloaded with several ways to construct the object.  Too, I am not sure how templating the class would work with template value parameters (vice type params).
>
> Given that templating the constructor is well-supported syntactically, the suggestion to use a factory function seems inelegant.
>
> Thanks for listening and suggestions.
> James

This works fro the struct case:

struct A {
	string s;
	int n;
	this( string s) { this.s = s; }
	this( int k)( int n) { this.n = n - k; }
}
void main()
{
    A a = void;
    a.__ctor!3(42);
}

Not sure about the class case but you can probably concoct something similar.