Re: Make sure lifetime of helper structs is less than owning struct

[Ali Çehreli via Digitalmars-d-learn]

On 11/15/21 1:58 PM, WebFreak001 wrote:
> On Monday, 15 November 2021 at 19:24:56 UTC, Sebastiaan Koppe wrote:
>> On Monday, 15 November 2021 at 15:56:57 UTC, WebFreak001 wrote:
>>> is this currently possible or maybe possible with DIP1000?
>>
>> Yes it is. But besides `-dip1000` and `@safe`, it requires the use of
>> a pointer:
>>
>> ```D
>> @safe:
>>
>> struct ByChunk {
>>     FileReader* r;
>>     void popFront() {}
>> }
>>
>> struct FileReader {
>>     ByChunk byChunk() return scope {
>>         return ByChunk(&this);
>>     }
>> }
>>
>> void main() {
>>     ByChunk helper;
>>     {
>>            auto file = FileReader();
>>         helper = file.byChunk;  // Error: address of variable `file`
>> assigned to `helper` with longer lifetime
>>     }
>>     helper.popFront;
>> }
>> ```
>
> awesome, such a simple solution! Also saves me the pain of copying the
> struct data and avoiding copy constructor and stuff by using a pointer.
I don't see how it solves the problem. Sebastiaan Koppe might have 
intended to allocate the object dynamically with 'new' as well?

import std.stdio;

@safe:

struct ByChunk {
  FileReader* r;

  ~this() {
    writeln(__FUNCTION__);
  }

  void popFront() {
    writeln("popFront on ", r);
  }
}

struct FileReader {
  ~this() {
    writeln(__FUNCTION__, " on ", &this);
  }

  ByChunk byChunk() return scope {
    return ByChunk(&this);
  }
}

void main() {
  ByChunk helper;
  {
    auto file = new FileReader();  // <-- NOTE new
    helper = file.byChunk;
  }
  writeln("back in main");
  helper.popFront;
}

This is the current output:

deneme.ByChunk.~this
back in main
popFront on 7F12B377E000
deneme.ByChunk.~this
deneme.FileReader.~this on 7F12B377E000

But without that 'new', FileReader is destroyed before popFront:

deneme.ByChunk.~this
deneme.FileReader.~this on 7FFD8231D4B8   <-- BAD
back in main
popFront on 7FFD8231D4B8
deneme.ByChunk.~this

I think I am misunderstanding something here. :)

Ali