Manually allocating a File

[Adam D. Ruppe]

On Tuesday, 20 February 2018 at 14:56:54 UTC, Chris M. wrote:
> I'm doing this mainly for experimentation, but the following 
> piece of code gives all sorts of errors.

so important note: this will perform worse than the automatic 
allocation, which just puts it down in-place. You should 
basically never `new` or `malloc` a `File` struct. (in fact, i 
think we should @disable new on it! yes, D can do that!)


>     auto f = cast(File *) malloc(File.sizeof);
>     *f = File("test.txt", "r");

Two notes here: 1) it will call the destructor on *f, which is 
uninitialized data and 2) the destructor that actually wants to 
run will have to wait until the GC reaps it, which is also not 
ideal. The segfault you see is in #2 here, though I'm not sure 
exactly what is causing it, could be out-of-order, could be some 
other corruption, idk.

Generally, when manually allocating D objects, first malloc it, 
then copy the `.init` value over, before casting it to the other 
type - do all this as bytes so it doesn't trigger postblits, 
destructors, etc like a real struct. The `std.conv.emplace` 
function from phobos does this and its source code can help you 
see how in a few cases.

>     (*f).readln.writeln;

That * is unnecessary btw, in D you can

f.readln

and it will see it is a pointer to struct and dereference it for 
you.


> I could have sworn I've done something similar recently and it 
> worked, unfortunately I can't remember what the case was.

With other types, you can do this, but File is designed to be 
used in-place...