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digitalmars.D.learn - Creating fixed array on stack

reply Andrey <saasecondbox yandex.ru> writes:
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.
Jan 11
parent reply Dgame <r.schuett.1987 gmail.com> writes:
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); } ----
Jan 11
next sibling parent Johan Engelen <j j.nl> writes:
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
Jan 11
prev sibling parent reply Andrey <saasecondbox yandex.ru> writes:
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?
Jan 12
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
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.
Jan 12
parent Seb <seb wilzba.ch> writes:
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
Jan 12