EXC_BAD_ACCESS calling c++ class method from NSTimer - c++

Question

Running into issues trying to mix c++ and objective-c when building cocoa app using Xcode4.
The issue is when I use NSTimer to call handleFrame function, which calls a virtual function of a class.
Here is what i am trying to do:
1. Create a monitor;
2. Create a handler;
3. Assign handler to monitor (init function)
4. Call monitor->update() that is expected to call handler's virtual method.
5. Code works as expected in applicationDidFinishLaunching function, however NSTimer is causing EXC_BAD_ACCESS exception in handleFrame.
//
// AppDelegate.h
// Concept5
//
#import <Cocoa/Cocoa.h>
#include "monitor.h"
#include "Derived.h"
#interface AppDelegate : NSObject <NSApplicationDelegate>
{
Monitor *monitor;`enter code here`
NSTimer *gameTimer;
}
#property (assign) IBOutlet NSWindow *window;
- (void)handleFrame:(NSTimer *)timer;
#end
AppDelegate implementation (.mm)
//
// AppDelegate.mm
// Concept5
//
#import "AppDelegate.h"
#implementation AppDelegate
- (void)dealloc
{
[super dealloc];
}
- (id) init {
self = [super init];
if(self) {
monitor = new Monitor();
}
return self;
}
- (void)handleFrame:(NSTimer *)timer {
monitor->update();
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
// Insert code here to initialize your application
Derived derived;
monitor->init(derived);
monitor->update();
gameTimer = [[NSTimer scheduledTimerWithTimeInterval:1
target:self
selector:#selector(handleFrame:)
userInfo:nil
repeats:YES] retain];
monitor->update();
}
#end
//
// Monitor.cpp
// Concept5
//
#include "Monitor.h"
void Monitor::init (Base& handler)
{
_handler = &handler;
}
void Monitor::update()
{
if (_handler != NULL)
{
_handler->speak(); // <-- EXC_BAD_ACCESS exception.
}
}
//
// Monitor.h
// Concept5
#ifndef __Concept5__Monitor__
#define __Concept5__Monitor__
#include <iostream>
#include "Base.h"
class Monitor
{
private:
Base* _handler;
public:
void init (Base& handler);
void update();
};
#endif /* defined(__Concept5__Monitor__) */
//
// Base.cpp
// Concept5
#include "Base.h"
void Base::speak()
{
std::cout << "Base speaks" << std::endl;
}
//
// Base.h
// Concept5
#ifndef __Concept5__Base__
#define __Concept5__Base__
#include <iostream>
class Base
{
public:
virtual void speak();
};
#endif /* defined(__Concept5__Base__) */
//
// Derived.cpp
// Concept5
#include "Derived.h"
void Derived::speak()
{
std::cout << "Derived speaks" << std::endl;
}
//
// Derived.h
// Concept5
//
#ifndef __Concept5__Derived__
#define __Concept5__Derived__
#include <iostream>
#include "Base.h"
class Derived : public Base
{
public:
void speak();
};
#endif /* defined(__Concept5__Derived__) */

Answer 1

I have never used Objective-C, but the following looks like an issue:
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
Derived derived;
monitor->init(derived);
//....
}
Since Derived is a local variable, the scope will not go beyond the applicationDidFinishLaunching function. Thus calling init() (which takes a pointer), will be holding onto an invalid object when the above function returns.
If this were C++, the solution is to make sure the lifetime of the object is sufficient. The usual solutions are:
1) Make the object global, or
2) Create the object dynamically using new, or
3) Create a smart pointer (probably std::shared_ptr) and use that instead of raw pointers.

Answer 2

I'm not an Objective-C expert, but EXC_BAD_ACCESS mean you are trying to access something with a bad pointer, likely a nil pointer.
Since your timer is calling an INSTANCE method in stead of a CLASS method on a Monitor instance, you better have a monitor instance when your timer fires. My guess is that you don't. If I was more of an Objective-C person, I could probably look at your code & see this, but as it is I would have to run your code to tell. But I'd bet that is what is wrong.
That all being said, it is a risky idea to call timers with instance methods, unless you are absolutely sure the instance will still be around & you know what you're doing. It is safer to only call class methods on a timer.