C++: Undefined reference to instance in Singleton class
c++, linker, singleton, static, undefined-reference
Solution
You must define the static instance, not just declare it. The definition creates the actual object you refer to.
In your `cpp` file, add the line:
boost::shared_ptr<ImpFactory> ImpFactory::mInstance;
Problem
I'm currently trying to implement a factory as a singleton. I practically used the textbook example of the Singleton pattern. Here's the .h file: ``` namespace oxygen{ class ImpFactory{ public: static boost::shared_ptr<ImpFactory> GetInstance(); private: static boost::shared_ptr<ImpFactory> mInstance; }; ``` and here's the .cpp file: ``` #include "impfactory.h" using namespace oxygen; using namespace boost; shared_ptr<ImpFactory> ImpFactory::GetInstance(){ if (mInstance.get() == 0) mInstance = shared_ptr<ImpFactory>(new ImpFactory()); return mInstance; } ``` The code compiles, but I get a linker error: ../../lib/oxygen/liboxygen.so.3.2.4: undefined reference to `oxygen::ImpFactory::mInstance' This currently has three students stumped. Any ideas?