c++ template singleton static pointer initialization in header file

Question

What is wrong with this implementation in header file?

template 
class Singleton
{
public:
    static T* getInstance() 
    {
        if (m         


        

Answer 1

You can fix your error by adding a definition for the m_instance member after the class definition.

template
T* Singleton::m_instance = nullptr;

For class templates, it's OK to add the definition of static members within the header itself, and won't lead to ODR violations.

But, as others have suggested, it's best to change you getInstance() definition to

static T& getInstance() 
{
    static T instance;
    return instance;
}

C++11 even guarantees that the creation of the function local static variable instance will be thread-safe.