Passing struct to function

[Michał]

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?