Question

Consider the following construct:

//! Templated singleton.
/*!
    Template wrapper enforcing the singleton behavior.
*/
template <class T>
class TSingleton
{
private:    
    //! Singleton instance pointer.
    static T* instance;
    //! private constructor.
    TSingleton() { }
    //! private empty copy constructor.
    TSingleton(const TSingleton<T>& sourceObject) {}

public:
    //! Static singleton instance getter.
    static T* GetInstance()
    {
        if (instance == 0)
            instance = new T();
        return instance;
    }

};

template <class T> T* TSingleton<T>::instance = 0;

This template class and the definition of the static instance are written in the same header file. For a non-template class, this causes a link-time error due to multiple symbols being defined for the instance static member. It seems intuitive for this to happen with templates as well, thus one must separate the definition and put it in a .cpp file. But templates are usually declared and defined in header-like files. What is it that allows this syntax to be valid and functional for template classes?

There a wikipedia link on this, but it does not provide a clear explanation on what happens in case of template classes.

Answer 1

This works because [basic.def.odr]/5 explicitly allowed templates to be duplicated:

There can be more than one definition of a class type (Clause 9), enumeration type (7.2), inline function with external linkage (7.1.2), class template (Clause 14), non-static function template (14.5.6), static data member of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for which some template parameters are not specified (14.7, 14.5.5) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. …

The requirements are quite lengthy, so I won’t duplicate them here, but essentially they state that each duplicate definition must be identical (otherwise the program has undefined behaviour).