File-like option where the "file contents" comes from a string?

[Dennis]

On Wednesday, 27 November 2024 at 01:12:23 UTC, Andy Valencia 
wrote:
> Again and again for testing I run into how nice it would be to 
> have an open "File" which has its contents set by the unit test 
> code

I don't know what situation you're in so this may not be 
applicable, but I personally rarely use mocks, because I write my 
code as `pure` as possible and push dependencies on the file 
system / environment / user interface etc. to the top. For 
example, instead of:

```D
void main(string[] args)
{
     writeln(lineCount(File(args[1], "r")));
}

int lineCount(File f) => f.byLine.walkLength;

unittest
{
     // Annoyingly need to create file with test input
     assert(lineCount(StringFile("Good\nday")) == 2);
}
```

You can also write it like:

```D
void main(string[] args)
{
     writeln(lineCount(cast(string) read(args[1])));
}

int lineCount(string f) pure => f.splitter('\n').walkLength;

unittest
{
     assert(lineCount("Good\nday") == 2);
}
```

This is a toy example. Again, you may have constraints that don't 
allow structuring your code like that, but it's worth considering.