I am working on a Multithreaded system here's my code
class demo is defined in .h file
when the loop from the main function is executed second time the COMMENT1 below takes the previous value
doesn't closing handle closes the thread?
int threadentry(void* data)
{
demo* inst=(demo*) data;
cout << "Value of inst "<<hex << &inst<< endl;
string request;
cin>>request;
if(request==play)
{
inst->play;
cout << "Value of inst "<<hex << &inst<< endl;
// COMMENT1 here when the thread is executed second time from the main it is taking previous value
}
}
int main()
{
while(1)
{
demo* inst=new demo();
cout << "Value of inst "<<hex << &inst<< endl; //value is coming different from above
HANDLE threads;
DWORD threadId1;
if ((threads = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)threadentry,
(void *)inst, 0, &threadId1)) == NULL)
return -1;
//here is some Processing of data and after processing I close the handle
CloseHandle(threads);
delete inst;
system("pause");
}
}
No -- closing a handle to a thread does not destroy the thread itself. The thread should exit (either by calling ExitThread or by just returning from the thread function) when it's finished doing its job.
In emergencies, you can use TerminateThread to kill a thread, but that should be reserved for true emergencies -- it can leave the process in an unstable state, so it should generally be avoided, and if you have to use it, you probably want to shut down the process as soon afterwards as possible.
Also note that in a program that uses the standard library, it's not really safe to use CreateThread directly -- you should call _beginthread or _beginthreadex instead. These do some setup to allow thread-safe use of standard library functions that use static storage (e.g., strtok and mktime, but there are quite a few more).
Drop all those "(type)foo" casts, they are forcing the compiler to accept things that in reality don't fit. You will have to fix a few errors there by replacing things with the proper type. For the context pointer passed to the thread, the conversion from demo* to void* is implicit. The correct cast to reverse this is static_cast<demo*>(data). If you want, you can use the a static cast for the implicit conversion, too. There are missing return values in functions, too, the only case that is allowed is in main(). The reason I mention th s is that formally, anything can happen in your program, because these things cause undefined behaviour.
Then, you are outputting the "value of inst" but actually outputting the address of local variables called "inst", which is something different. This probably just adds to your confusion.
Now, coming to your problem, CloseHandle() does not stop the thread. It only releases your handle. What you want is WaitForSingleObject() or one of its brethren instead.
Related
When I run several std::threads in parallell and need to cancel other threads in a deferred manner if one thread fails I use a std::atomic<bool> flag:
#include <thread>
#include <chrono>
#include <iostream>
void threadFunction(unsigned int id, std::atomic<bool>& terminated) {
srand(id);
while (!terminated) {
int r = rand() % 100;
if (r == 0) {
std::cerr << "Thread " << id << ": an error occured.\n";
terminated = true; // without this line we have to wait for other thread to finish
return;
}
std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
}
int main()
{
std::atomic<bool> terminated = false;
std::thread t1(&threadFunction, 1, std::ref(terminated));
std::thread t2(&threadFunction, 2, std::ref(terminated));
t1.join();
t2.join();
std::cerr << "Both threads finished.\n";
int k;
std::cin >> k;
}
However now I am reading about std::stop_sourceand std::stop_token.
I find that I can achieve the same as above by passing both a std::stop_sourceby reference and std::stop_token by value to the thread function?
How would that be superior?
I understand that when using std::jthread the std::stop_token is very convenient if I want to stop threads from outside the threads.
I could then call std::jthread::request_stop() from the main program.
However in the case where I want to stop threads from a thread is it still better?
I managed to achieve the same thing as in my code using std::stop_source:
void threadFunction(std::stop_token stoken, unsigned int id, std::stop_source source) {
srand(id);
while (!stoken.stop_requested()) {
int r = rand() % 100;
if (r == 0) {
std::cerr << "Thread " << id << ": an error occured.\n";
source.request_stop(); // without this line we have to wait for other thread to finish
return;
}
std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
}
int main()
{
std::stop_source source;
std::stop_token stoken = source.get_token();
std::thread t1(&threadFunction, stoken, 1, source);
std::thread t2(&threadFunction, stoken, 2, source);
t1.join();
t2.join();
std::cerr << "Both threads finished.\n";
int k;
std::cin >> k;
}
Using std::jthread would have resulted in more compact code:
std::jthread t1(&threadFunction, 1, source);
std::jthread t2(&threadFunction, 2, source);
But that did not seem to work.
It didn't work because std::jthread has a special feature where, if the first parameter of a thread-function is a std::stop_token, it fills that token in by an internal stop_source object.
What you ought to do is only pass a stop_source (by value, not by reference), and extract the token from it within your thread function.
As for why this is better than a reference to an atomic, there are a myriad of reasons. The first being that stop_source is a lot safer than a bare reference to an object whose lifetime is not under the local control of the thread function. The second being that you don't have to do std::ref gymnastics to pass parameters. This can be a source of bugs since you might accidentally forget to do that in some place.
The standard stop_token mechanism has features beyond just requesting and responding to a stop. Since the response to a stop happens at an arbitrary time after issuing it, it may be necessary to execute some code when the stop is actually requested rather than when it is responded to. The stop_callback mechanism allows you to register a callback with a stop_token. This callback will be called in the thread of the stop_source::request_stop call (unless you register the callback after the stop was requested, in which case it's called right when you register it). This can be useful in limited cases, and it's not simple code to write yourself. Especially when all you have is an atomic<bool>.
And then there's simple readability. Passing a stop_source tells you exactly what is going on without having to even see the name of a parameter. Passing an atomic<bool> tells you very little from just the typename; you have to look at the parameter name or its usage in the function to know that it is for halting the thread.
Apart from being more expressive and communicating intentions better, stop_token and friends achieve something really important for jthread. To understand it you have to consider its destructor which looks something like this:
~jthread()
{
if(joinable())
{
// Not only user code, but the destructor as well
// will let your callback know it's time to go.
request_stop();
join();
}
}
by encapsulating a stop_source, jthread facilitates what is called cooperative cancellation. As you've also noted, you never have to pass the stop_token to a jthread, just provide a callback that accepts the token as its first parameter. What happens next is that the class can detect that your callback accepts a stop token and pass a token to its internal stop source when calling it.
What does this mean for cooperative cancellation? Safer termination of course! Since jthread will always attempt to join on destruction, it now has the means to prevent endless loops and deadlocks where two or more threads wait for each other to finish. By using stop_token your code can make sure that it can safely join when it's time to go.
However in the case where I want to stop threads from a thread is it still better?
Now regarding the feature you are requesting, that's what C# calls "linked cancellation". Yes, there are requests and discussions to add a parameter in the jthread constructor so that it can refer to an external stop source, but that's not yet available (and has many implications). Doing something similar purely with stop tokens would require a stop_callback to tie all cancellations together, but still it could be suboptimal (as shown in the link). The bottom line is that jthread needs stop_token, but in some cases you may not need jthread, especially if the following solution does not appeal to you:
stop_source ssource;
std::stop_callback cb {ssource.get_token(), [&] {
t1.request_stop();
t2.request_stop();
}};
ssource.request_stop(); // This stops boths threads.
The good news is that if you don't fall into the suboptimal pattern described in the link (i.e. you don't need an asynchronous termination), then this functionality is easy to abstract into a utility, something like:
auto linked_cancellations = [](auto&... jthreads) {
stop_source s;
return std::make_pair(s, std::stop_callback{
s.get_token(), [&]{ (jthreads.request_stop(), ...); }});
};
which you'd use as
auto [stop_source, cb] = linked_cancellations(t1, t2);
// or as many thread objects as you want to link ^^^
stop_source.request_stop(); // Stops all the threads that you linked.
Now if you want to control the linked threads from within the thread, I'd use the initial pattern (std::atomic<bool>), since having a callback with both a stop token and a stop source is somewhat confusing.
Does it ever make sense to check if this is null?
Say I have a class with a method; inside that method, I check this == NULL, and if it is, return an error code.
If this is null, then that means the object is deleted. Is the method even able to return anything?
Update: I forgot to mention that the method can be called from multiple threads and it may cause the object to be deleted while another thread is inside the method.
Does it ever make sense to check for this==null? I found this while doing a code review.
In standard C++, it does not, because any call on a null pointer is already undefined behavior, so any code relying on such checks is non-standard (there's no guarantee that the check will even be executed).
Note that this holds true for non-virtual functions as well.
Some implementations permit this==0, however, and consequently libraries written specifically for those implementations will sometimes use it as a hack. A good example of such a pair is VC++ and MFC - I don't recall the exact code, but I distinctly remember seeing if (this == NULL) checks in MFC source code somewhere.
It may also be there as a debugging aid, because at some point in the past this code was hit with this==0 because of a mistake in the caller, so a check was inserted to catch future instances of that. An assert would make more sense for such things, though.
If this == null then that means the object is deleted.
No, it doesn't mean that. It means that a method was called on a null pointer, or on a reference obtained from a null pointer (though obtaining such a reference is already U.B.). This has nothing to do with delete, and does not require any objects of this type to have ever existed.
Your note about threads is worrisome. I'm pretty sure you have a race condition that can lead to a crash. If a thread deletes an object and zeros the pointer, another thread could make a call through that pointer between those two operations, leading to this being non-null and also not valid, resulting in a crash. Similarly, if a thread calls a method while another thread is in the middle of creating the object, you may also get a crash.
Short answer, you really need to use a mutex or something to synchonize access to this variable. You need to ensure that this is never null or you're going to have problems.
I know that this is old but I feel like now that we're dealing with C++11-17 somebody should mention lambdas. If you capture this into a lambda that is going to be called asynchronously at a later point in time, it is possible that your "this" object gets destroyed before that lambda is invoked.
i.e passing it as a callback to some time-expensive function that is run from a separate thread or just asynchronously in general
EDIT: Just to be clear, the question was "Does it ever make sense to check if this is null" I am merely offering a scenario where it does make sense that might become more prevalent with the wider use of modern C++.
Contrived example:
This code is completely runable. To see unsafe behavior just comment out the call to safe behavior and uncomment the unsafe behavior call.
#include <memory>
#include <functional>
#include <iostream>
#include <future>
class SomeAPI
{
public:
SomeAPI() = default;
void DoWork(std::function<void(int)> cb)
{
DoAsync(cb);
}
private:
void DoAsync(std::function<void(int)> cb)
{
std::cout << "SomeAPI about to do async work\n";
m_future = std::async(std::launch::async, [](auto cb)
{
std::cout << "Async thread sleeping 10 seconds (Doing work).\n";
std::this_thread::sleep_for(std::chrono::seconds{ 10 });
// Do a bunch of work and set a status indicating success or failure.
// Assume 0 is success.
int status = 0;
std::cout << "Executing callback.\n";
cb(status);
std::cout << "Callback Executed.\n";
}, cb);
};
std::future<void> m_future;
};
class SomeOtherClass
{
public:
void SetSuccess(int success) { m_success = success; }
private:
bool m_success = false;
};
class SomeClass : public std::enable_shared_from_this<SomeClass>
{
public:
SomeClass(SomeAPI* api)
: m_api(api)
{
}
void DoWorkUnsafe()
{
std::cout << "DoWorkUnsafe about to pass callback to async executer.\n";
// Call DoWork on the API.
// DoWork takes some time.
// When DoWork is finished, it calls the callback that we sent in.
m_api->DoWork([this](int status)
{
// Undefined behavior
m_value = 17;
// Crash
m_data->SetSuccess(true);
ReportSuccess();
});
}
void DoWorkSafe()
{
// Create a weak point from a shared pointer to this.
std::weak_ptr<SomeClass> this_ = shared_from_this();
std::cout << "DoWorkSafe about to pass callback to async executer.\n";
// Capture the weak pointer.
m_api->DoWork([this_](int status)
{
// Test the weak pointer.
if (auto sp = this_.lock())
{
std::cout << "Async work finished.\n";
// If its good, then we are still alive and safe to execute on this.
sp->m_value = 17;
sp->m_data->SetSuccess(true);
sp->ReportSuccess();
}
});
}
private:
void ReportSuccess()
{
// Tell everyone who cares that a thing has succeeded.
};
SomeAPI* m_api;
std::shared_ptr<SomeOtherClass> m_data = std::shared_ptr<SomeOtherClass>();
int m_value;
};
int main()
{
std::shared_ptr<SomeAPI> api = std::make_shared<SomeAPI>();
std::shared_ptr<SomeClass> someClass = std::make_shared<SomeClass>(api.get());
someClass->DoWorkSafe();
// Comment out the above line and uncomment the below line
// to see the unsafe behavior.
//someClass->DoWorkUnsafe();
std::cout << "Deleting someClass\n";
someClass.reset();
std::cout << "Main thread sleeping for 20 seconds.\n";
std::this_thread::sleep_for(std::chrono::seconds{ 20 });
return 0;
}
FWIW, I have used debugging checks for (this != NULL) in assertions before which have helped catch defective code. Not that the code would have necessarily gotten too far with out a crash, but on small embedded systems that don't have memory protection, the assertions actually helped.
On systems with memory protection, the OS will generally hit an access violation if called with a NULL this pointer, so there's less value in asserting this != NULL. However, see Pavel's comment for why it's not necessarily worthless on even protected systems.
Your method will most likely (may vary between compilers) be able to run and also be able to return a value. As long as it does not access any instance variables. If it tries this it will crash.
As others pointed out you can not use this test to see if an object has been deleted. Even if you could, it would not work, because the object may be deleted by another thread just after the test but before you execute the next line after the test. Use Thread synchronization instead.
If this is null there is a bug in your program, most likely in the design of your program.
I'd also add that it's usually better to avoid null or NULL. I think the standard is changing yet again here but for now 0 is really what you want to check for to be absolutely sure you're getting what you want.
This is just a pointer passed as the first argument to a function (which is exactly what makes it a method). So long as you're not talking about virtual methods and/or virtual inheritance, then yes, you can find yourself executing an instance method, with a null instance. As others said, you almost certainly won't get very far with that execution before problems arise, but robust coding should probably check for that situation, with an assert. At least, it makes sense when you suspect it could be occuring for some reason, but need to track down exactly which class / call stack it's occurring in.
I know this is a old question, however I thought I will share my experience with use of Lambda capture
#include <iostream>
#include <memory>
using std::unique_ptr;
using std::make_unique;
using std::cout;
using std::endl;
class foo {
public:
foo(int no) : no_(no) {
}
template <typename Lambda>
void lambda_func(Lambda&& l) {
cout << "No is " << no_ << endl;
l();
}
private:
int no_;
};
int main() {
auto f = std::make_unique<foo>(10);
f->lambda_func([f = std::move(f)] () mutable {
cout << "lambda ==> " << endl;
cout << "lambda <== " << endl;
});
return 0;
}
This code segment faults
$ g++ -std=c++14 uniqueptr.cpp
$ ./a.out
Segmentation fault (core dumped)
If I remove the std::cout statement from lambda_func The code runs to completion.
It seems like, this statement f->lambda_func([f = std::move(f)] () mutable { processes lambda captures before member function is invoked.
I've worked a decent amount with threading in C on linux and now I'm trying to do the same but with c++ on Windows, but I'm having trouble with printing to the standard output. In the function the thread carries out I have:
void print_number(void* x){
int num = *(static_cast<int*> (x));
std::cout << "The number is " << num << std::endl;
}
wrapped in a loop that creates three threads. The problem is that although everything gets printed, the threads seem to interrupt each other between each of the "<<"'s.
For example, the last time I ran it I got
The number is The number is 2The number is 3
1
When I was hoping for each on a separate line. I'm guessing that each thread is able to write to the standard output after another has written a single section between "<<"s. In C, this wasn't a problem because the buffer wasn't flushed until everything I needed the write was there, but that's not the case now I don't think. Is this a case of a need for a mutex?
In C++, we first of all would prefer to take arguments as int*. And then, we can just lock. In C++11:
std::mutex mtx; // somewhere, in case you have other print functions
// that you want to control
void print_number(int* num) {
std::unique_lock<std::mutex> lk{mtx}; // RAII. Unlocks when lk goes out of scope
std::cout << "The number is " << *num << std::endl;
}
If not C++11, there's boost::mutex and boost::mutex::scoped_lock that work the same way and do the same thing.
Your C example worked by accident; printf and the like aren't atomic either.
This is indeed a case for a mutex. I typically allocate it static function locally. E.g.:
void atomic_print(/*args*/) {
static MyMutex mutex;
mutex.acquire();
printf(/*with the args*/);
mutex.release();
}
I can't seem to tell why I can't stop the timer I have created using DeleteTimerQueue or any vesrion of it, such as DeleteTimerQueueTimer or Ex. Of course I fill in the arguments as needed.
At first I wanted to stop/delete the timer at a certain condition, but I have since removed that condition(an if loop) to see if Delete is working and it is not.
I am calling the Delete within the callback and I have two timers I want to stop at the same time the moment one of them accomplishes a task (that if Loop I'm talking about).
Anyway here is how I create the timers
BOOL champ1_Success = ::CreateTimerQueueTimer(
&baseAttackTimerHandler,
NULL,
callBaseAttackC1,
this,
0,
(DWORD)(1000/attackSpeed),
WT_EXECUTEDEFAULT);
BOOL champ2_Success = ::CreateTimerQueueTimer(
&baseAttackTimerHandler,
NULL,
callBaseAttackC2,
this,
0,
(DWORD)(1000/attackSpeed),
WT_EXECUTEDEFAULT);
They work great! If only I could stop them.
Within their callbacks I call
void CALLBACK Controller::callBaseAttackC1(void* lpParametar,
BOOLEAN TimerOrWaitFired)
{
// This is used only to call QueueTimerHandler
// Typically, this function is static member of CTimersDlg
Controller* obj = (Controller*) lpParametar;
if (!obj->yi->isAlive() && !obj->skarner->isAlive()){
if (!DeleteTimerQueueTimer(baseAttackTimerHandler, NULL, NULL)){
printf("Base Attack Timer failed (%d)\n", GetLastError());
obj->out << "Base A Timer Failed " << GetLastError() << endl;
}
//CloseHandle(baseAttackTimerHandler);
}
obj->basicAttackC1_TimeHandler();
}
Note the
if (!DeleteTimerQueueTimer(baseAttackTimerHandler, NULL, NULL)){
printf("Base Attack Timer failed (%d)\n", GetLastError());
obj->out << "Base A Timer Failed " << GetLastError() << endl;
}
I figure that calling the Delete within the callback would mean the timer only executes once, but it is not. It just keeps going. Another thing to note, and I'm not sure whether its good , bad or what it means but I'm have both different timers using the same Pointer to a handle, HANDL object I've created (HANDLE baseAttackTimerHandler = NULL;). I've tried making them of their own unique but it made no difference so I opted to have them use the same. BTW, DeleteTimerQueue Keeps failing when called. Error Code 87?
Bottom line is I really need them to stop, any idea what I am doing wrong or tricks to get them too?
EDIT: So I got it to work like this...
if (!DeleteTimerQueueTimer(NULL, baseAttackTimerHandler_C1, NULL)){
printf("2-Base Attack Timer failed (%d)\n", GetLastError());
obj->out << "2-Base A Timer Failed " << GetLastError() << endl;
}
but I had to make different handles for each timer to prevent error. (C1 and C2). Even though it puts out an error the first time, it stops after one execution.
However, putting it under my if loop prevents it from working. Going to have to look into my if loop.
EDIT:
Figured out what was wrong with my if loop. Since the callback seems to have to create an object of the class its in (Controller), its different than the one I initialized and am working with. So even though I do stuff with Controller con I make a Controller obj. Results in obj not the same as con and my if loop never results in true. Going to have to figure this one out. If there were a way to make the callback not be static and still use it in CreateTimer I could call con.
EDIT: Just used a strcut to make an object to set the obj equal to con indirectly. Works. I'm almost done, The timer stops but now my pop up message keeps getting spammed. I think I'm done with this thread though.
Yes, you have to use a different handle for each timer. Your original code has:
CreateTimerQueueTimer(&baseAttackTimerHandler ...);
CreateTimerQueueTimer(&baseAttackTimerHandler ...);
The second call was overwriting the handle from the first call. So when you called DeleteTimerQueueTimer to delete the timer, the handle you passed would delete the second timer but not the first.
Your solution is correct: use a different variable to hold each timer handle. That shouldn't be surprising. How can you expect to store two different handles in a single handle value?
Does it ever make sense to check if this is null?
Say I have a class with a method; inside that method, I check this == NULL, and if it is, return an error code.
If this is null, then that means the object is deleted. Is the method even able to return anything?
Update: I forgot to mention that the method can be called from multiple threads and it may cause the object to be deleted while another thread is inside the method.
Does it ever make sense to check for this==null? I found this while doing a code review.
In standard C++, it does not, because any call on a null pointer is already undefined behavior, so any code relying on such checks is non-standard (there's no guarantee that the check will even be executed).
Note that this holds true for non-virtual functions as well.
Some implementations permit this==0, however, and consequently libraries written specifically for those implementations will sometimes use it as a hack. A good example of such a pair is VC++ and MFC - I don't recall the exact code, but I distinctly remember seeing if (this == NULL) checks in MFC source code somewhere.
It may also be there as a debugging aid, because at some point in the past this code was hit with this==0 because of a mistake in the caller, so a check was inserted to catch future instances of that. An assert would make more sense for such things, though.
If this == null then that means the object is deleted.
No, it doesn't mean that. It means that a method was called on a null pointer, or on a reference obtained from a null pointer (though obtaining such a reference is already U.B.). This has nothing to do with delete, and does not require any objects of this type to have ever existed.
Your note about threads is worrisome. I'm pretty sure you have a race condition that can lead to a crash. If a thread deletes an object and zeros the pointer, another thread could make a call through that pointer between those two operations, leading to this being non-null and also not valid, resulting in a crash. Similarly, if a thread calls a method while another thread is in the middle of creating the object, you may also get a crash.
Short answer, you really need to use a mutex or something to synchonize access to this variable. You need to ensure that this is never null or you're going to have problems.
I know that this is old but I feel like now that we're dealing with C++11-17 somebody should mention lambdas. If you capture this into a lambda that is going to be called asynchronously at a later point in time, it is possible that your "this" object gets destroyed before that lambda is invoked.
i.e passing it as a callback to some time-expensive function that is run from a separate thread or just asynchronously in general
EDIT: Just to be clear, the question was "Does it ever make sense to check if this is null" I am merely offering a scenario where it does make sense that might become more prevalent with the wider use of modern C++.
Contrived example:
This code is completely runable. To see unsafe behavior just comment out the call to safe behavior and uncomment the unsafe behavior call.
#include <memory>
#include <functional>
#include <iostream>
#include <future>
class SomeAPI
{
public:
SomeAPI() = default;
void DoWork(std::function<void(int)> cb)
{
DoAsync(cb);
}
private:
void DoAsync(std::function<void(int)> cb)
{
std::cout << "SomeAPI about to do async work\n";
m_future = std::async(std::launch::async, [](auto cb)
{
std::cout << "Async thread sleeping 10 seconds (Doing work).\n";
std::this_thread::sleep_for(std::chrono::seconds{ 10 });
// Do a bunch of work and set a status indicating success or failure.
// Assume 0 is success.
int status = 0;
std::cout << "Executing callback.\n";
cb(status);
std::cout << "Callback Executed.\n";
}, cb);
};
std::future<void> m_future;
};
class SomeOtherClass
{
public:
void SetSuccess(int success) { m_success = success; }
private:
bool m_success = false;
};
class SomeClass : public std::enable_shared_from_this<SomeClass>
{
public:
SomeClass(SomeAPI* api)
: m_api(api)
{
}
void DoWorkUnsafe()
{
std::cout << "DoWorkUnsafe about to pass callback to async executer.\n";
// Call DoWork on the API.
// DoWork takes some time.
// When DoWork is finished, it calls the callback that we sent in.
m_api->DoWork([this](int status)
{
// Undefined behavior
m_value = 17;
// Crash
m_data->SetSuccess(true);
ReportSuccess();
});
}
void DoWorkSafe()
{
// Create a weak point from a shared pointer to this.
std::weak_ptr<SomeClass> this_ = shared_from_this();
std::cout << "DoWorkSafe about to pass callback to async executer.\n";
// Capture the weak pointer.
m_api->DoWork([this_](int status)
{
// Test the weak pointer.
if (auto sp = this_.lock())
{
std::cout << "Async work finished.\n";
// If its good, then we are still alive and safe to execute on this.
sp->m_value = 17;
sp->m_data->SetSuccess(true);
sp->ReportSuccess();
}
});
}
private:
void ReportSuccess()
{
// Tell everyone who cares that a thing has succeeded.
};
SomeAPI* m_api;
std::shared_ptr<SomeOtherClass> m_data = std::shared_ptr<SomeOtherClass>();
int m_value;
};
int main()
{
std::shared_ptr<SomeAPI> api = std::make_shared<SomeAPI>();
std::shared_ptr<SomeClass> someClass = std::make_shared<SomeClass>(api.get());
someClass->DoWorkSafe();
// Comment out the above line and uncomment the below line
// to see the unsafe behavior.
//someClass->DoWorkUnsafe();
std::cout << "Deleting someClass\n";
someClass.reset();
std::cout << "Main thread sleeping for 20 seconds.\n";
std::this_thread::sleep_for(std::chrono::seconds{ 20 });
return 0;
}
FWIW, I have used debugging checks for (this != NULL) in assertions before which have helped catch defective code. Not that the code would have necessarily gotten too far with out a crash, but on small embedded systems that don't have memory protection, the assertions actually helped.
On systems with memory protection, the OS will generally hit an access violation if called with a NULL this pointer, so there's less value in asserting this != NULL. However, see Pavel's comment for why it's not necessarily worthless on even protected systems.
Your method will most likely (may vary between compilers) be able to run and also be able to return a value. As long as it does not access any instance variables. If it tries this it will crash.
As others pointed out you can not use this test to see if an object has been deleted. Even if you could, it would not work, because the object may be deleted by another thread just after the test but before you execute the next line after the test. Use Thread synchronization instead.
If this is null there is a bug in your program, most likely in the design of your program.
I'd also add that it's usually better to avoid null or NULL. I think the standard is changing yet again here but for now 0 is really what you want to check for to be absolutely sure you're getting what you want.
This is just a pointer passed as the first argument to a function (which is exactly what makes it a method). So long as you're not talking about virtual methods and/or virtual inheritance, then yes, you can find yourself executing an instance method, with a null instance. As others said, you almost certainly won't get very far with that execution before problems arise, but robust coding should probably check for that situation, with an assert. At least, it makes sense when you suspect it could be occuring for some reason, but need to track down exactly which class / call stack it's occurring in.
I know this is a old question, however I thought I will share my experience with use of Lambda capture
#include <iostream>
#include <memory>
using std::unique_ptr;
using std::make_unique;
using std::cout;
using std::endl;
class foo {
public:
foo(int no) : no_(no) {
}
template <typename Lambda>
void lambda_func(Lambda&& l) {
cout << "No is " << no_ << endl;
l();
}
private:
int no_;
};
int main() {
auto f = std::make_unique<foo>(10);
f->lambda_func([f = std::move(f)] () mutable {
cout << "lambda ==> " << endl;
cout << "lambda <== " << endl;
});
return 0;
}
This code segment faults
$ g++ -std=c++14 uniqueptr.cpp
$ ./a.out
Segmentation fault (core dumped)
If I remove the std::cout statement from lambda_func The code runs to completion.
It seems like, this statement f->lambda_func([f = std::move(f)] () mutable { processes lambda captures before member function is invoked.