Remove or specify what a “variable length struct” is

[Bolpat]

The array spec reads:

Static arrays with 0 elements are useful as the last member of a variable length struct, or as the degenerate case of a template expansion.

(Highlight by me)

Neither the array spec nor the struct spec mention it. A Google Search yields only one result and it’s the quoted paragraph.

So please, either

• remove its mention (if it never was supported and support isn’t planned),
• mention it’s a future feature,
• mention it’s no longer supported (if it’s not supported anymore),
• specify how it works, or
• put a link in the array spec to where it’s documented

I can’t do that myself because I don’t know which bullet point is the right one.

[dkorpel]

Variable length structs are an idiom from C where you inline an array with runtime length as the last member of a struct. So instead of dereferencing a pointer to get to the elements:

struct String
{
    size_t length;
    char* chars;
}

You just put them right after all the statically sized members of the struct. Since you can't create an array char[n] based on a runtime n, you can use 0 like the spec suggests:

struct String
{
    size_t length;
    char[0] chars;
}

Then you dynamically allocate such a struct with malloc(String.sizeof + length) and access the contents like so:

char[] getSlice(String* s) => s.chars.ptr[0 .. s.length];

An example of such a struct in real usage is:

https://github.com/dlang/dmd/blob/master/compiler/src/dmd/root/stringtable.d#L50-L55

But that one uses (&this+1) to locate the chars instead of accessing a char[0] member.

Apparently C has a language feature for this: https://en.wikipedia.org/wiki/Flexible_array_member#cite_note-1
There are no plans to add such a feature to D at the moment.

[Bolpat]

@dkorpel, I know what a variable-length struct is in C, but the spec neither links to or references the C spec, so a naive reader must assume it’s a D feature (and it is), but it’s nowhere specified. The only information you gave me is that it’s actually supported. That narrows it down to:

• specify how it works, or
• put a link in the array spec to where it’s documented, or
• (new) put a link in the array spec to the C language spec.

[dkorpel]

Let's remove it, I wouldn't consider it a 'best practice' anyways. I once translated a C variable length array to a 0-size static array, and just got an index out of bounds error on usage (you have to remember to add .ptr on each index). dmd itself doesn't even use it. It's more of a footgun than a feature.