Passing struct to function

[Steven Schveighoffer]

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