I'm learning how to implement a thread safe singleton pattern in c++11 and later.
#include <iostream>
#include <memory>
#include <mutex>
class Singleton
{
public:
static Singleton& get_instance();
void print();
private:
static std::unique_ptr<Singleton> m_instance;
static std::once_flag m_onceFlag;
Singleton(){};
Singleton(const Singleton& src);
Singleton& operator=(const Singleton& rhs);
};
std::unique_ptr<Singleton> Singleton::m_instance = nullptr;
std::once_flag Singleton::m_onceFlag;
Singleton& Singleton::get_instance(){
std::call_once(m_onceFlag, [](){m_instance.reset(new Singleton());});
return *m_instance.get();
};
void Singleton::print(){
std::cout << "Something" << std::endl;
}
int main(int argc, char const *argv[])
{
Singleton::get_instance().print();
return 0;
}
The code compiles fine but when executing i receive the following Exception.
terminate called after throwing an instance of 'std::system_error'
what(): Unknown error -1
Aborted
i tried to debug the program with gdb. It seems that the exception is thrown when calling std::call_once. I'm not sure about what's happening but i assume that the lambda expression failed to create the object.
A second question. Is there a way to know what unknown error codes actually mean ? i think a -1 will not help much when trying to identify the problem.
Thanks for your help.
This happens because you did not compile with the -pthread flag and are attempting to use utilities from the native threading library on your system.
As an alternative look into the following change to the singleton pattern definition in your example referred to as the "Meyers Singleton"
Singleton& Singleton::get_instance(){
static Singleton instance;
return instance;
}
This is thread safe, and will cause the instance variable to only be initialized once. This wikipedia article has a good explanation of how it possibly works under the hood https://en.wikipedia.org/wiki/Double-checked_locking. It is good to let the compiler arrange code for you whenever possible. Also as noted in the comments this question also has useful information about the above Is Meyers' implementation of the Singleton pattern thread safe?
Related
I have been trying to figure out why this is happening and maybe it is just due to inexperience at this point but could really use some help.
When I run my code, which is compiled into a DLL using C++20, I get that a debug assertion has failed with the expression being __acrt_first_block == header.
I narrowed down where the code is failing, but the weird part is that it runs just fine when I change the Init(std::string filePath function signature to not contain the parameter. The code is below and hope someone can help.
Logger.h
#pragma once
#include "../Core.h"
#include <memory>
#include <string>
#include "spdlog/spdlog.h"
namespace Ruby
{
class RUBY_API Logger
{
public:
static void Init(std::string filePath);
inline static std::shared_ptr<spdlog::logger>& GetCoreLogger() { return coreLogger; }
inline static std::shared_ptr<spdlog::logger>& GetClientLogger() { return clientLogger; }
private:
static std::shared_ptr<spdlog::logger> coreLogger;
static std::shared_ptr<spdlog::logger> clientLogger;
};
}
Logger.cpp
namespace Ruby
{
std::shared_ptr<spdlog::logger> Logger::coreLogger;
std::shared_ptr<spdlog::logger> Logger::clientLogger;
void Logger::Init(std::string filePath)
{
std::string pattern{ "%^[%r][%n][%l]: %v%$" };
auto fileSink = std::make_shared<spdlog::sinks::basic_file_sink_mt>(filePath, true);
// Setup the console and file sinks
std::vector<spdlog::sink_ptr> coreSinks;
coreSinks.push_back(std::make_shared<spdlog::sinks::stdout_color_sink_mt>());
coreSinks.push_back(fileSink);
// Bind the sinks to the core logger.
coreLogger = std::make_shared<spdlog::logger>("RUBY", begin(coreSinks), end(coreSinks));
// Set the Patterns for the sinks
coreLogger->sinks()[0]->set_pattern(pattern);
coreLogger->sinks()[1]->set_pattern(pattern);
// Tell spdlog to flush the file loggers on trace or worse message (can be changed if necessary).
coreLogger->flush_on(spdlog::level::trace);
// Set the default level of the logger
coreLogger->set_level(spdlog::level::trace);
// Do the same for the client logger
std::vector<spdlog::sink_ptr> clientSinks;
clientSinks.push_back(std::make_shared<spdlog::sinks::stdout_color_sink_mt>());
clientSinks.push_back(fileSink);
clientLogger = std::make_shared<spdlog::logger>("APP", begin(clientSinks), end(clientSinks));
clientLogger->sinks()[0]->set_pattern(pattern);
clientLogger->sinks()[1]->set_pattern(pattern);
clientLogger->flush_on(spdlog::level::trace);
clientLogger->set_level(spdlog::level::trace);
}
}
Entrypoint.h
#pragma once
#ifdef RB_PLATFORM_WINDOWS
extern Ruby::Application* Ruby::CreateApplication();
int main(int argc, char** argv)
{
Ruby::Logger::Init("../Logs/Recent_Run.txt");
RB_CORE_INFO("Initialized the logger.");
auto app = Ruby::CreateApplication();
app->Run();
delete app;
return 0;
}
#else
#error Ruby only supports windows
#endif // RB_PLATFORM_WINDOWS
For anyone else who runs into a similar problem, here is how I fixed it.
Essentially the function signature for the Init() function was the problem. The std::string parameter was causing the debug assertion to fire, my best guess as of right now was because of move semantics but that part I am still not sure on. So there are a couple of ways that I found to fix this.
Method 1:
Make the parameter a const char*. I don't quite like this approach as it then relies on C style strings and if you are trying to write a program in modern C++, this is a huge step backwards.
Method 2:
Make the parameter a const std::string&. Making it a const reference to a string prevents the move semantics (again as far as I know) and the assertion no longer fires. I prefer this fix as it keeps the program in modern C++.
I hope this helps anyone who has similar issues, and be careful with statics and move semantics.
This question already has answers here:
Start thread with member function
(5 answers)
Closed 3 years ago.
When I compile the code below I get the following error. help me...!
Errors:
Error C2276 '&': illegal operation on bound member function expression
Error C3867 'CCore::Run': non - standard syntax; use '&' to create a pointer to member
I'm not sure if the program gives me an error.
I want to run the "Run" function of the Core class.
Core.cpp file contains only the functions created by the compiler.
I'm learning English so I'm not good yet. So please understand that the whole code.
// main.cpp
#include "Core.h"
#include <thread>
int main()
{
// The conditions below have been success
if (CCore::GetInstance().Init())
{
// The code below fails to compile.
// Error C3867 'CCore::Run': non - standard syntax; use '&' to create a pointer to member
thread main_thread(CCore::GetInstance().Run);
// Error C2276 '&': illegal operation on bound member function expression
thread main_thread(&CCore::GetInstance().Run);
main_thread.join();
}
return 0;
}
// Core.h
#pragma once
#include "Singleton.h"
#include <iostream>
using namespace std;
class CCore : public Singleton<CCore>
{
public:
CCore();
~CCore();
bool Init();
void Run();
};
// Singleton.h
#pragma once
template<typename T>
class Singleton
{
protected:
Singleton()=default;
~Singleton()=default;
public:
static T& GetInstance()
{
static T instance;
return instance;
}
Singleton(const Singleton&) = delete;
Singleton& operator=(const Singleton) = delete;
};
The compiler tells you the problem: Run() isn't a free function. It's a method, i.e. a function bound to an instance of an object. You have several options, but generally you either let the compiler synthesize a runnable for you, or write a free function yourself:
Let the compiler do all the work: guess what, it can make singletons for you so how cool is that?!
std::thread main_thread([]{
static CCore myCore;
myCore.Run();
});
You want to access that core? Sure!
std::future<CCore*> coreWhenDone = std::async([]{
static CCore myCore;
myCore.Run();
return &myCore;
});
Better yet, the core would provide some result, so that instead of accessing it directly when it's done, you could get its result (e.g. an int or std::vector<double> or whatever that core is computing)/
Let the compiler do some of the work:
std::thread main_thread([]{ CCore::GetInstance().Run(); });
Split the work between yourself and the compiler:
std::thread main_thread(std::bind(&CCore::Run, &CCore::GetInstance()));
Do all the work yourself:
void runCoreRun() {
CCore::GetInstance().Run();
}
...
std::thread main_thread(&runCoreRun);
I've been porting some c++ app from Visual Studio 2013 to Visual Studio 2017. Aside from the plethora of new warnings that I had to fix, the compilation and linking went okay.
However, when running the app, it 'stalled' when trying to re-enter the constructor of a singleton (when successive function calls form a loop back to the constructor). It seems that this behaviour was okay in VS2013, but is no longer valid in VS2017. There is no error message.
I'm aware of all the bad things related to singletons, and that there should at least not be loops. The question is not there.
Is there a way to tell the VS2017 compiler that I'd like to shoot myself in the foot, and allow the same behaviour that was there in VS2013?
I don't have access to the code that causes this behaviour because it comes from a third-party library, this is why I can't 'just fix it', unfortunately.
Here is an example which works in VS2013, but doesn't work in VS2017:
main.cpp
#include "Singleton.h";
int
main( void )
{
std::cout << "let's do this!" << std::endl;
int two = Singleton::GetReference().getTwo();
std::cout << "ok" << std::endl;
return 0;
}
Singleton.h
#pragma once
class Stuff;
class Singleton
{
public:
static Singleton& GetReference();
int getTwo() { return 2; }
private:
Singleton();
Stuff* stuff;
};
Singleton.cpp
#include "Singleton.h"
#include "Stuff.h"
Singleton&
Singleton::GetReference() {
static Singleton theInstance;
return theInstance;
}
Singleton::Singleton()
{
stuff = new Stuff();
}
Stuff.h
#pragma once
class Stuff
{
public:
Stuff();
private:
int two;
};
Stuff.cpp
#include "Stuff.h"
#include "Singleton.h"
Stuff::Stuff()
{
two = Singleton::GetReference().getTwo();
}
In the code above, when step-by-step debugging, the first time we get on the line static Singleton theInstance; will work as expected, but the second time, a F11 will go to the file thread_safe_statics.cpp, into the method extern "C" void __cdecl _Init_thread_header(int* const pOnce). A Shift+F11 will exit the method and the program will wait indefinitely at the line specified (observed when pausing the program from the debugger).
PS
This issue probably occurs in Visual Studio 2015 too, as the documentation linked from the accepted answer mentions VS2015.
/Zc:threadSafeInit-
The general "Conformance" page is MSDN: Conformance, which details which new features you can disable.
I needed the code for sizedDealloc, where my new compiler was creating a sized new operator for a library which broke older compiled expectations.
As this is a compile flag, at least some of the code would be in your control, and you should be able to unravel the beast.
The constructor Stuff::Stuff is calling a function on an incompletely constructed object.
That would create "Undefined behavior". If the value "2" is not set till the end of the constructor (for example).
Probably the Singleton needs to be split into 2, one which delivers the early static data (e.g. 2).
The second which delivers the held object Stuff. Stuff would only rely on the first, which would break the deadlock.
Alternatively, a second constructor to Stuff which told it which object to use, and was called from the Singleton::Singleton
The MSDN article to disable "Magic Statics" MSDN : disable threadsafe static initialization
i have implemented a singleton (static version) in C++. I know all the controversy about this pattern and potential thread-safety issues, but i am curious why this exact implementation won't halt. The program never quits, it remains in a deadlock state at the end.
singleton.h:
#pragma once
#include <thread>
#include <atomic>
class Singleton
{
public:
static Singleton& getInstance();
private:
std::thread mThread;
std::atomic_bool mRun;
Singleton();
~Singleton();
void threadFoo();
};
singleton.cpp
#include "singleton.h"
Singleton& Singleton::getInstance()
{
static Singleton instance;
return instance;
}
Singleton::Singleton()
{
mRun.store(true);
mThread = std::thread(&Singleton::threadFoo, this);
}
Singleton::~Singleton()
{
mRun.store(false);
if(mThread.joinable())
mThread.join();
}
void Singleton::threadFoo()
{
while(mRun)
{
}
}
main.cpp
#include "singleton.h"
int main()
{
Singleton::getInstance();
return 0;
}
What I already know:
the thread terminates
the main thread is stuck in the join
it has something to do with the static, if i make the constructor public and create an instance of Singleton in main() it will correctly terminate.
Using Visual Studio 2012. Thanks for your advice.
On the main thread, after main() terminates, the CRT acquires the exit lock and calls your static instance destructor, which waits for your background thread to exit.
On the background thread, after your thread function terminates, the CRT attempts to acquire the exit lock to do some thread termination work. This blocks forever because the exit lock is held by the main thread, which is waiting for this thread to exit.
It's a simple deadlock that's caused by the CRT implementation. The bottom line is that you can't await thread termination in a static instance destructor on Windows.
I've traced it down to void __cdecl _lock(int locknum) inside mlock.c. When main() ends, the main thread goes there and enters critical section EnterCriticalSection( _locktable[locknum].lock );. Then Singleton destructor gets called and the other thread tries to enter the same critical section, but can't, and so it starts waiting for main thread to leave the critical section. Main thread, in turn, waits for the other thread. So I guess it's a bug.
See [basic.start.term] in the Standard:
If there is a use of a standard library object or function not
permitted within signal handlers (18.10) that does not happen before
(1.10) completion of destruction of objects with static storage
duration and execution of std::atexit registered functions (18.5), the
program has undefined behavior. [Note: If there is a use of an object
with static storage duration that does not happen before the object’s
destruction, the program has undefined behavior. Terminating every
thread before a call to std::exit or the exit from main is sufficient,
but not necessary, to satisfy these requirements. These requirements
permit thread managers as static-storage-duration objects. —end note ]
Ok thank you all for your hints. Apparently this pattern implementation results in a deadlock on VC++.
After doing some further research i found this implementation based on C++11 mechanics which is working in VC++.
singleton.h
#pragma once
#include <thread>
#include <atomic>
#include <memory>
#include <mutex>
class Singleton
{
public:
static Singleton& getInstance();
virtual ~Singleton();
private:
static std::unique_ptr<Singleton> mInstance;
static std::once_flag mOnceFlag;
std::thread mThread;
std::atomic_bool mRun;
Singleton();
void threadFoo();
};
singleton.cpp
#include "singleton.h"
std::unique_ptr<Singleton> Singleton::mInstance = nullptr;
std::once_flag Singleton::mOnceFlag;
Singleton& Singleton::getInstance()
{
std::call_once(mOnceFlag, [] { mInstance.reset(new Singleton); });
return *mInstance.get();
}
Singleton::Singleton()
{
mRun.store(true);
mThread = std::thread(&Singleton::threadFoo, this);
}
Singleton::~Singleton()
{
mRun.store(false);
if(mThread.joinable())
mThread.join();
}
void Singleton::threadFoo()
{
while(mRun.load())
{
}
}
UPDATE
It looks like Microsoft is aware of this issue. In the VC++ forums a user named "dlafleur" reported this post:
https://connect.microsoft.com/VisualStudio/feedback/details/747145
This deadlock bug is the same as in
std::thread::join() hangs if called after main() exits when using VS2012 RC
and it is not fixed in Visual Studio 2013.
It seems to be fixed in Visual Studio 2015 and above, at least for this specific example.
I am designing a thread-safe singleton class in C++ on Linux using boost::call_once.
I got the following compiler error:
/tmp/ccLanvZI.o: In function `SingleTon::getInstance()':
singleTon.cpp:(.text._ZN9SingleTon11getInstanceEv[SingleTon::getInstance()]+0x1b): undefined reference to `SingleTon::p'
/tmp/ccLanvZI.o: In function `SingleTon::makeInstance()':
singleTon.cpp:(.text._ZN9SingleTon12makeInstanceEv[SingleTon::makeInstance()]+0x21): undefined reference to `SingleTon::SingleTon()'
singleTon.cpp:(.text._ZN9SingleTon12makeInstanceEv[SingleTon::makeInstance()]+0x2c): undefined reference to `SingleTon::p'
collect2: ld returned 1 exit status
make: *** [singlton] Error 1
After seeing some posts, I still do not know how to handle this error.
How do I change the void (SingleTon::)() to void (*)()?
#include <iostream>
#include <pthread.h>
#include <boost/thread/mutex.hpp>
#include <boost/thread/once.hpp>
#include <boost/bind.hpp>
#include <boost/threadpool.hpp>
#include <boost/thread/thread.hpp>
boost::once_flag flag = BOOST_ONCE_INIT;
class SingleTon
{
private:
static SingleTon *p;
SingleTon();
SingleTon(const SingleTon&);
SingleTon& operator=(const SingleTon&);
void makeInstance();
public:
SingleTon* getInstance();
~SingleTon()
{
if (p)
delete p;
}
};
void SingleTon::makeInstance()
{
p = new SingleTon;
return;
}
SingleTon* SingleTon::getInstance()
{
boost::call_once( makeInstance , flag); // error !!!
return p;
}
int main()
{
SingleTon *s;
s = SingleTon::getInstance();
return 0;
}
The error indicates boost::call_once doesn't work on member function pointers. This is because you forgot to make both makeInstance and getInstance static.
Why have you written a destructor? Which instance do you plan to destroy? Oh wait, there's only one, cos it's a singleton, so in that case why does the destructor try to delete the instance? Infinitely recursive failure.
Singletons smell anyway, avoid them.
As AardvarkSoup (tasty and nutritious) pointed out, you need makeInstance() and getInstance() to be static.
Also, you need to define SingleTon(), not just declare it, since it's used in makeInstance().
Finally, you need to initialise the static member p, like:
SingleTon* SingleTon::p(nullptr);
This makes the code compile. However, designing a thread-safe singleton class is non-trivial. See this question for piles of links to very full explanations as to why you should try and avoid using singletons.