June 14
Hi all,

I now really appreciate the power Ranges provide and am an avid consumer, but am only slowly becoming accustomed to implementing my own.

In the present problem, I am writing a binding to a C library (htslib) that provides many functions related to high-throughput sequencing files. One of these functions is for rapid indexed lookup into multi-GB files. The library provides a handle to an iterator which must be supplied to a "get next matching row" type function, which overall seems perfect for implementation as a range.  You can see my naive implementation here:

https://github.com/blachlylab/dhtslib/blob/master/source/dhtslib/tabix.d

Note that TabixIndexedFile::region returns an InputRange; in the original implementation, this Range preloaded the first record (the ctor called popFirst()), but ultimately I realized this was not workable because copies of the object would always be non-empty. In some ways, this problem is generalizable to all InputRanges that represent a file or record stream.


My problems now are at least twofold.

1. If I use the range, the destructor seems to be called many, many times. This is directly related to problem 2, below, but I would be interested to understand why this is happening generally. For example, see:
https://github.com/blachlylab/dhtslib/blob/master/test/tabix_gffreader.d
Here, when I create the range but do not consume it, the ctor and dtor are called once each, as expected. However, if I foreach(line; r) { } the destructor is called twice. If I reason through this, it is because use of the range created a copy to consume. (?) However, if instead, I writeln( r ), the destructor is called *five* times. I cannot understand the reason for this, unless it is black magic required by writeln().

I assume the (apparent) lack of parity between ctor and dtor is because the "default postblit" (which I figured out for a struct means an empty `this(this)` ctor) is called when a copy is made. My understanding is that I cannot disable the default postblit and still act as a range, correct? Should I be overloading this?

2. Directly related to the above, I need, when the range is consumed, to free() the underlying library's iterator handle. Naively, I had the destructor do this, but obviously with multiple calls to ~this I end up with an error free()'ing a pointer that is no longer alloc'd.  What is the correct way to handle this situation in D?

Other Range and destructor advice generally (e.g., "You should totally change your design or approach to X instead") is always welcomed.

James