digitalmars.D.learn - GC can collect object allocated in function, despite a pointer to the
- =?UTF-8?B?IuWyqeWAiSDmvqoi?= (55/55) Aug 16 2014 Hello, I've been interested in the D language for a few years
- Sean Kelly (4/4) Aug 16 2014 Interface and object variables are reference types--you don't
- =?UTF-8?B?IuWyqeWAiSDmvqoi?= (6/10) Aug 16 2014 Thank you for the help! I just removed the unnecessary
- Chris Cain (21/44) Aug 16 2014 // !!!!!!!!!!!!!!!
- =?UTF-8?B?IuWyqeWAiSDmvqoi?= (2/7) Aug 16 2014
Hello, I've been interested in the D language for a few years now, but only dabble. I have read TDPL and recently started reading D Cookbook. On the side, I started to write a small game in D, but ran into a problem. I would greatly appreciate any help! My design is a simple state machine. I have a module game.states, which includes: interface IState{ void handleEvents(); void renderFrame(); } IState *currentState; string nextState; void changeState(){ if(nextState != "WaitState" && nextState != "ExitState"){ auto newState = cast(IState) Object.factory("game.states."~nextState); import std.exception; enforce(newState); currentState = &newState; nextState = "WaitState"; } } class TitleState : IState{ void handleEvents(){ ... } void renderFrame(){ ... } } ... } and the game loop is of the following form: //init state nextState = "TitleState"; changeState(); //game loop while(nextState != "ExitState"){ currentState.handleEvents(); currentState.renderFrame(); changeState(); } However, it appears that the changeState function has a bug. I believe the problem is that when changeState returns, newState gets garbage collected, despite currentState pointing to it. I come from a C++ background, so I am not used to garbage collection. Normally the changeState function would explicitly free the old state, and allocate the new one. Am I correct that newState is liable to be collected after changeState returns? Is there an easy fix? Is my design fundamentally flawed within the context of garbage collection? If so, what kind of design would you recommend instead?
Aug 16 2014
Interface and object variables are reference types--you don't need the '*' to make them so. By adding the extra layer of indirection you're losing the only reference the GC can decipher to the currentState instance.
Aug 16 2014
Thank you for the help! I just removed the unnecessary indirection and it is working great! I was aware that interface and object variables are reference types, but it slipped my mind. I'm too used to the C++ way of things still :p On Saturday, 16 August 2014 at 22:41:45 UTC, Sean Kelly wrote:Interface and object variables are reference types--you don't need the '*' to make them so. By adding the extra layer of indirection you're losing the only reference the GC can decipher to the currentState instance.
Aug 16 2014
On Saturday, 16 August 2014 at 22:36:51 UTC, 岩倉 澪 wrote:void changeState(){ if(nextState != "WaitState" && nextState != "ExitState"){ auto newState = cast(IState) Object.factory("game.states."~nextState); import std.exception; enforce(newState);// !!!!!!!!!!!!!!!currentState = &newState;// !!!!!!!!!!!!!!!nextState = "WaitState"; } }However, it appears that the changeState function has a bug. I believe the problem is that when changeState returns, newState gets garbage collected, despite currentState pointing to it. I come from a C++ background, so I am not used to garbage collection. Normally the changeState function would explicitly free the old state, and allocate the new one. Am I correct that newState is liable to be collected after changeState returns? Is there an easy fix? Is my design fundamentally flawed within the context of garbage collection? If so, what kind of design would you recommend instead?This is actually not garbage collection. &newState is making a pointer to a reference that is located on the stack (that is, when you return from that function you now have a pointer that may at any time become overwritten and made invalid.) As it turns out, interfaces/classes in D are already reference types, so you can just do something like this: IState currentState; // reference to an IState void changeState(){ if(nextState != "WaitState" && nextState != "ExitState"){ auto newState = cast(IState) Object.factory("game.states."~nextState); import std.exception; enforce(newState); currentState = newState; // changed nextState = "WaitState"; } }
Aug 16 2014
I see now, makes sense. :) On Saturday, 16 August 2014 at 22:43:21 UTC, Chris Cain wrote:This is actually not garbage collection. &newState is making a pointer to a reference that is located on the stack (that is, when you return from that function you now have a pointer that may at any time become overwritten and made invalid.)
Aug 16 2014