Thread overview
Creating fixed array on stack
Jan 11
Andrey
Jan 11
Dgame
Jan 12
Andrey
Jan 12
Seb
January 11
Hi,
In C++ you can create a fixed array on stack:
> int count = getCount();
> int myarray[count];

In D the "count" is part of type and must be known at CT but in example it is RT.
How to do such thing in D? Without using of heap.
January 11
On Friday, 11 January 2019 at 14:46:36 UTC, Andrey wrote:
> Hi,
> In C++ you can create a fixed array on stack:
>> int count = getCount();
>> int myarray[count];
>
> In D the "count" is part of type and must be known at CT but in example it is RT.
> How to do such thing in D? Without using of heap.

You could try alloca:

----
import core.stdc.stdlib: alloca;

pragma(inline, true) auto stack(T, alias len)(void* p = alloca(T.sizeof * len)) {
    return (cast(T*) p)[0 .. len] = T.init;
}

void main() {
    import std.stdio: writeln;

    int size = 42;
    auto a = stack!(int, size);
    writeln(a);
    a[] = 2;
    writeln(a);
}
----
January 11
On Friday, 11 January 2019 at 15:23:08 UTC, Dgame wrote:
> On Friday, 11 January 2019 at 14:46:36 UTC, Andrey wrote:
>> Hi,
>> In C++ you can create a fixed array on stack:
>>> int count = getCount();
>>> int myarray[count];

Small correction: this is valid in C, but not in C++.

>> In D the "count" is part of type and must be known at CT but in example it is RT.
>> How to do such thing in D? Without using of heap.
>
> You could try alloca:

Indeed, but make sure you read up on the caveats of using alloca. I believe all use cases of VLA/alloca have better alternative implementation options.

-Johan

January 12
On Friday, 11 January 2019 at 15:23:08 UTC, Dgame wrote:
> On Friday, 11 January 2019 at 14:46:36 UTC, Andrey wrote:
>> Hi,
>> In C++ you can create a fixed array on stack:
>>> int count = getCount();
>>> int myarray[count];
>>
>> In D the "count" is part of type and must be known at CT but in example it is RT.
>> How to do such thing in D? Without using of heap.
>
> You could try alloca:
>
> ----
> import core.stdc.stdlib: alloca;
>
> pragma(inline, true) auto stack(T, alias len)(void* p = alloca(T.sizeof * len)) {
>     return (cast(T*) p)[0 .. len] = T.init;
> }
>
> void main() {
>     import std.stdio: writeln;
>
>     int size = 42;
>     auto a = stack!(int, size);
>     writeln(a);
>     a[] = 2;
>     writeln(a);
> }
> ----

Thank you.
But this requires using of C function "alloca". I think this cause some RT overhead (and in output asm code) in comparison with C++ variant. Or I'm not right?
January 12
On Saturday, 12 January 2019 at 08:10:47 UTC, Andrey wrote:
> But this requires using of C function "alloca". I think this cause some RT overhead (and in output asm code) in comparison with C++ variant. Or I'm not right?

alloca is a magical function; a compiler intrinsic that works the same way as the C built-in.
January 12
On Saturday, 12 January 2019 at 13:17:00 UTC, Adam D. Ruppe wrote:
> On Saturday, 12 January 2019 at 08:10:47 UTC, Andrey wrote:
>> But this requires using of C function "alloca". I think this cause some RT overhead (and in output asm code) in comparison with C++ variant. Or I'm not right?
>
> alloca is a magical function; a compiler intrinsic that works the same way as the C built-in.

Yup, specifically (at least for DMD) it is defined here:

https://github.com/dlang/druntime/blob/master/src/rt/alloca.d