digitalmars.D.learn - Passing struct to function

=?UTF-8?B?TWljaGHFgg==?= (31/31) Jun 13 2018 When I pass my struct to function something is going wrong. I

Steven Schveighoffer (8/50) Jun 13 2018 D allows moving any struct instance without calling postblit, as long as...

=?UTF-8?B?TWljaGHFgg==?= (9/60) Jun 13 2018 I need internal pointer because I want to implement vector(like

Steven Schveighoffer (21/85) Jun 13 2018 Hm... the only way to do it in D is to provide a function that checks

=?UTF-8?B?TWljaGHFgg==?= (3/24) Jun 13 2018 Thanks for an idea, I will try it.

=?UTF-8?B?TWljaGHFgg==?= <mmcomando gmail.com> writes:

When I pass my struct to function something is going wrong. I 
don't know how to fix it.

Code:
import std.stdio;


void print(ref Vector v, string s){
		writefln("%s==%s    %s", &v.x, v.ptr, s);
}

struct Vector {
	int x;
	int* ptr;

	this(this) {
		ptr = &x;
		print(this, "postblit");
	}
}

void someFunc(Vector t) {
	print(t, "in someFunc");
}

void main() {
	auto tmpA = Vector();
	tmpA.ptr = &tmpA.x;
	print(tmpA, "start");

	someFunc(tmpA);
}


Result on my machine:
7FFF7D70BC00==7FFF7D70BC00    start
7FFF7D70BBF0==7FFF7D70BBF0    postblit
7FFF7D70BBD0==7FFF7D70BBF0    in someFunc

In the last line pointers are not matching. I thought that 
postblit will do the thing but it is not the case. How to make 
'ptr' to be null or '&this.x' all the time?

Jun 13 2018

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 6/13/18 10:43 AM, Michał wrote:
 When I pass my struct to function something is going wrong. I don't know 
 how to fix it.
 
 Code:
 import std.stdio;
 
 
 void print(ref Vector v, string s){
          writefln("%s==%s    %s", &v.x, v.ptr, s);
 }
 
 struct Vector {
      int x;
      int* ptr;
 
      this(this) {
          ptr = &x;
          print(this, "postblit");
      }
 }
 
 void someFunc(Vector t) {
      print(t, "in someFunc");
 }
 
 void main() {
      auto tmpA = Vector();
      tmpA.ptr = &tmpA.x;
      print(tmpA, "start");
 
      someFunc(tmpA);
 }
 
 
 Result on my machine:
 7FFF7D70BC00==7FFF7D70BC00    start
 7FFF7D70BBF0==7FFF7D70BBF0    postblit
 7FFF7D70BBD0==7FFF7D70BBF0    in someFunc
 
 In the last line pointers are not matching. I thought that postblit will 
 do the thing but it is not the case. How to make 'ptr' to be null or 
 '&this.x' all the time?

D allows moving any struct instance without calling postblit, as long as 
the original is no longer used.

The optimizer is likely seeing here that the memory can be copied 
without calling postblit, because nobody is using tmpA after the call.

In general, you should NOT store an internal pointer in a struct, unless 
you allocate it on the heap.

-Steve

Jun 13 2018

=?UTF-8?B?TWljaGHFgg==?= <mmcomando gmail.com> writes:

On Wednesday, 13 June 2018 at 16:40:51 UTC, Steven Schveighoffer 
wrote:
 On 6/13/18 10:43 AM, Michał wrote:
 When I pass my struct to function something is going wrong. I 
 don't know how to fix it.
 
 Code:
 import std.stdio;
 
 
 void print(ref Vector v, string s){
          writefln("%s==%s    %s", &v.x, v.ptr, s);
 }
 
 struct Vector {
      int x;
      int* ptr;
 
      this(this) {
          ptr = &x;
          print(this, "postblit");
      }
 }
 
 void someFunc(Vector t) {
      print(t, "in someFunc");
 }
 
 void main() {
      auto tmpA = Vector();
      tmpA.ptr = &tmpA.x;
      print(tmpA, "start");
 
      someFunc(tmpA);
 }
 
 
 Result on my machine:
 7FFF7D70BC00==7FFF7D70BC00    start
 7FFF7D70BBF0==7FFF7D70BBF0    postblit
 7FFF7D70BBD0==7FFF7D70BBF0    in someFunc
 
 In the last line pointers are not matching. I thought that 
 postblit will do the thing but it is not the case. How to make 
 'ptr' to be null or '&this.x' all the time?

 D allows moving any struct instance without calling postblit, 
 as long as the original is no longer used.

 The optimizer is likely seeing here that the memory can be 
 copied without calling postblit, because nobody is using tmpA 
 after the call.

 In general, you should NOT store an internal pointer in a 
 struct, unless you allocate it on the heap.

 -Steve


I need internal pointer because I want to implement vector(like 
in C++) with 'small vector optimization', when i have internal 
pointer it is very easy and functions like 'add' don't have 
additional checks.

If storing internal pointers is forbidden do you know some way to 
implement 'small vector optimization' without additional checks 
in 'add' function?

Jun 13 2018

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 6/13/18 1:08 PM, Michał wrote:
 On Wednesday, 13 June 2018 at 16:40:51 UTC, Steven Schveighoffer wrote:
 On 6/13/18 10:43 AM, Michał wrote:
 When I pass my struct to function something is going wrong. I don't 
 know how to fix it.

 Code:
 import std.stdio;


 void print(ref Vector v, string s){
          writefln("%s==%s    %s", &v.x, v.ptr, s);
 }

 struct Vector {
      int x;
      int* ptr;

      this(this) {
          ptr = &x;
          print(this, "postblit");
      }
 }

 void someFunc(Vector t) {
      print(t, "in someFunc");
 }

 void main() {
      auto tmpA = Vector();
      tmpA.ptr = &tmpA.x;
      print(tmpA, "start");

      someFunc(tmpA);
 }


 Result on my machine:
 7FFF7D70BC00==7FFF7D70BC00    start
 7FFF7D70BBF0==7FFF7D70BBF0    postblit
 7FFF7D70BBD0==7FFF7D70BBF0    in someFunc

 In the last line pointers are not matching. I thought that postblit 
 will do the thing but it is not the case. How to make 'ptr' to be 
 null or '&this.x' all the time?

 D allows moving any struct instance without calling postblit, as long 
 as the original is no longer used.

 The optimizer is likely seeing here that the memory can be copied 
 without calling postblit, because nobody is using tmpA after the call.

 In general, you should NOT store an internal pointer in a struct, 
 unless you allocate it on the heap.

 -Steve

 
 
 I need internal pointer because I want to implement vector(like in C++) 
 with 'small vector optimization', when i have internal pointer it is 
 very easy and functions like 'add' don't have additional checks.
 
 If storing internal pointers is forbidden do you know some way to 
 implement 'small vector optimization' without additional checks in 'add' 
 function?

Hm... the only way to do it in D is to provide a function that checks 
whether the small vector optimization is in play, and return a 
pointer/slice to itself.

With D it is possible to alias the getter function that provides the 
actual data to allow code to look nicer.

For example (crude example):

struct Vector(T)
{
    bool svo; // small vector optimization
    union
    {
       T[4] local;
       T[] heap;
    }
    inout(T)[] get() inout { return svo ? local[], heap; }
    alias get this;
    ... // implement specialized append, concat operators, etc.
}

Now, you can use Vector as if it were an array, and it just works.

-Steve

Jun 13 2018

=?UTF-8?B?TWljaGHFgg==?= <mmcomando gmail.com> writes:

On Wednesday, 13 June 2018 at 17:37:44 UTC, Steven Schveighoffer 
wrote:

 Hm... the only way to do it in D is to provide a function that 
 checks whether the small vector optimization is in play, and 
 return a pointer/slice to itself.

 With D it is possible to alias the getter function that 
 provides the actual data to allow code to look nicer.

 For example (crude example):

 struct Vector(T)
 {
    bool svo; // small vector optimization
    union
    {
       T[4] local;
       T[] heap;
    }
    inout(T)[] get() inout { return svo ? local[], heap; }
    alias get this;
    ... // implement specialized append, concat operators, etc.
 }

 Now, you can use Vector as if it were an array, and it just 
 works.

 -Steve

Thanks for an idea, I will try it.

Jun 13 2018

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digitalmars.D.learn - Passing struct to function