Threads not garbage collected ?

[Alex via Digitalmars-d]

On Wednesday, 22 February 2017 at 05:39:50 UTC, rikki cattermole 
wrote:
> On 22/02/2017 6:28 PM, Alex wrote:
>> import core.thread;
>>
>> class A
>> {
>>     Thread mThread;
>>     bool mStopped;
>>
>>     this()
>>     {
>>         mThread = new Thread(&run);
>>         mThread.start();
>>     }
>>
>>     void run()
>>     {
>>         while (!mStopped)
>>         {
>>             //do stuff
>>         }
>>     }
>>     ~this()
>>     {
>>         mStopped = true;
>>         mThread.join();
>>     }
>> }
>>
>>
>>
>> void main()
>> {
>>     auto a = new A;
>>     delete a;        //need this or the program hangs
>> }
>>
>> In both gdc and dmd I need to use manually delete this object 
>> or the
>> program is blocked after main. Is by design ?
>> It seems undesirable, as the thread can be many layers of 
>> encapsulation
>> down, and they all need manual deletes.
>
> Well, you never told the program to stop without delete.
>
> The thread that you start with (calls the main function) 
> doesn't actually have to stay alive throughout the program 
> running, surprise!
>
> Anyway, if you do want something that will stop try:
>
> import core.thread;
> import core.time;
> import std.stdio;
>
> class Foo : Thread {
>         bool keepRunning;
>
>         this() {
>                 keepRunning = true;
>                 super(&run);
>         }
>
>         private void run() {
>                 while(keepRunning) {
>                         sleep(1.seconds);
>                         writeln("iterate");
>                 }
>         }
> }
>
> void main() {
>         Foo foo = new Foo;
>         foo.start;
>
>         Thread.sleep(3.seconds);
>         foo.keepRunning = false;
> }

That is interesting, I did not know that the main thread could 
exit without returning control to the calling process.
Maybe this should be the case if we were writing in raw C against 
OS threading calls, but in this case I clearly have a thread 
Object not just an OS Handle, and I think most people would have 
an expectation that the garbage collector looks after deleting 
things in a garbage collected language when all the references 
are gone.

I have also tested throwing an exception in main, and yes the 
program hangs. This is even more of a problem because this 
undesirable behaviour may only be observed under exceptional 
circumstances.