On 1/4/22 9:48 AM, Paul Backus wrote:
> On Tuesday, 4 January 2022 at 17:01:41 UTC, Amit wrote:

>> I need a File (or a general IO interface) that reads from an
>> in-memory buffer, similar to python's `StringIO` or go's
>> `strings.Reader`.
>>
>> How can I achieve that?

I don't think it exists in the standard library. So, I had to write this for work manually. Instead of using File on the interfaces, I created a Storage interface that implemented everything I did with a File: open, close, writeln, seek, etc.

interface Storage {
  // ...
}

And I had two versions of it:

class FileStorage : Storage {
  // ...
}

class InMemoryStorage : Storage {
  ubyte[] buffer;
  // ...
}

Worked like a charm after fixing a number of bugs. (I wish it were open source.)

> Probably the easiest way to do it is to have your parser take a generic
> [range][1] as its argument instead of a `File`.

Makes sense but in my case File was everywhere so it felt better to abstract it away.

> For example, here's a function that parses an integer from an input range:

In my case, it would have to be a RandomAccessRange because the file format had self-references through offsets.

Ali

On Tue, Jan 04, 2022 at 05:01:41PM +0000, Amit via Digitalmars-d-learn wrote:
> Hi!
> 
> I wrote a text parser that takes a File argument and parses that file's contents. Now I would like to write a unit-test for that parser. I need a File (or a general IO interface) that reads from an in-memory buffer, similar to python's `StringIO` or go's `strings.Reader`.
> 
> How can I achieve that?
> 
> Example of what I have in mind:
> 
> ```d
> unittest {
>     string fakeContents = "foo\nbar\nbaz";
>     File f = createFileFromString(fakeContents);
>     assert(myParse(f) == MyParsedObject(...));
> }
> ```

It's very easy, just have your text parser take File as a template argument, defaulted to std.file.File, that your unittest(s) can override with a custom type containing the needed methods. For example:

	auto myParser(File = std.stdio.File)(File input) {
		... // read input as usual
	}

	unittest {
		struct FakeFile {
			string contents = "...";
			void[] rawRead(void[] buf) {
				... // simulate a file read here
			}
			... // add whatever other File methods you might need here
		}
		FakeFile f;
		auto output = myParser(f);
		...
	}

	void main() {
		auto input = File("...", "r");
		auto output = myParser(input); // `File` defaults to std.stdio.File
		...
	}


T

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