Summary:
Can a typesafe D variadic function, or D variadic template pass its
parameters to a C variadic function?
Background:
I maintain a library binding [0] to htslib, a high-performance and very
widely used C library for high-throughput sequencing (hts) data files.
We use this internally and haven't polished it for a release on the
announce forum or biomed twitter, etc. yet.
In the course of upgrading it to support the latest API/ABI (htslib-1.10
/ so.3), I need to add support for several new functions.
One of these is a variadic function with the signature:
`int sam_hdr_add_line(sam_hdr_t *h, const char *type, ...);`
Of course, we can call this directly from D with hardcoded parameters.
However, one of the focuses of our library is "bindings + wrappers" to
make this feel more like native D. Thus, we want a variadic function to
which we may pass D strings (it is also a struct member function).
With help from Herringway on IRC, we came up with a solution using mixin:
```
/// Add a single line to an existing header
auto addLine(T...)(RecordType type, T kvargs)
if(kvargs.length > 0 && isSomeString!(T[0]))
{
static assert (kvargs.length %2 == 0); // K-V pairs => even
number of variadic args
string varargMagic(size_t len)
{
string args = "sam_hdr_add_line(this.h, type.ptr, ";
for(int i=0; i<len; i++)
args ~= "toStringz(kvargs[" ~ i.to!string ~ "]), ";
args ~= "null)";
return args;
}
return mixin(varargMagic(kvargs.length));
}
```
Interestingly, compilation fails if the mixin consists only of the
comma-separated parameters ("Comma expression" [1])
Question:
If a variadic template, despite presenting to the user a "dynamic
array", MUST know its parameter list at compile-time, is there a way
(other than with mixins as shown) to pass this parameter list to
extern(C) linkage function with variadic parameters?
(bonus question: if yes, can it be done with typesafe variadic function
where I believe parameter list is known at either compile time OR
runtime, depending on how called)
Thanks all in advance
[0] dhtslib
https://code.dlang.org/packages/dhtslib
https://github.com/blachlylab/dhtslib
[1] https://dlang.org/spec/expression.html#expression
