Why undefined referenced to mMutex in my singleton class [duplicate] - c++

Question

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Undefined reference to static class member
(9 answers)
Closed 8 years ago.
I have implemented a Meyer's singleton pattern. And I try to do some stuff testing it in a multithreading environment. Here is how I implemented the class in C++.
#include<thread>
#include<mutex>
class singleton {
public:
static singleton& instance() {
std::lock_guard<std::mutex> lck(mtx);
static singleton *my = new singleton;
return *my;
}
private:
static std::mutex mtx;
singleton() {}
~singleton() {}
singleton(const singleton &) {}
singleton& operator=(const singleton &) {}
};
But when I compile this with g++ -std=c++11 singleton.cpp -o singleton -lpthread, it says
/tmp/ccfEBnmN.o: In function `singleton::instance()':
singleton.cpp(.text._ZN11singleton12instanceEv[_ZN11singelton12instanceEv]+0x10):
undefined reference to `singleton::mtx' collect2: error: ld returned 1 exit status
I understand this might be some problem in design, since without initializing the first singleton instance how could we get the mtx. If in this case, my question comes to how to implement a thread safe singleton class based on my code? How do we initialized the mtx in my singleton class?
I know there is a traditional way to create a singleton pattern by maintaining a static pointer points to the singleton class. But indeed that is not "so" thread safe. Even applying the double check mechanism in the instance() method.

Answer 1

You've declared mtx in singleton, but failed to define it, which has to be outside of singleton, something like this:
std::mutex singleton::mtx;
This should be in a source file, not a header though (otherwise you'll get multiple definition errors when/if you include the header in more than one source file).
Second point: what you're showing is not a Meyer's singleton pattern at all. For a Meyer's singleton, you define a static instance of the singleton inside the function:
static singleton &instance() {
static singleton my;
return my;
}
Using this, it's not clear that you really need a mutex at all.

Answer 2

Your static data member mtx needs a definition. Place this in your source file:
std::mutex singleton::mtx;
More accurately, std::lock_guard<std::mutex> lck(mtx) odr-uses mtx so its definition is required. You get a link-time error when the linker cannot resolve the reference.
Also note that in C++11 the initialization of an object with static storage duration is thread safe, you don't need a mutex.