Re: Question regarding a Singleton template
On Aug 20, 3:28 pm, DavidA <dandbn...@talktalk.net> wrote:
I am writing a C++ Singleton template based on some of the examples I
have seen. There is one detail I do not understand. In one example I
have seen, the Singleton 'instance' pointer is declared in a .h file
//! static class member initialisation.
template <typename T> T* CSingleton<T>::m_instance = NULL;
Now, in a non-templated Singleton, I would expect the instance pointer
to be declared in the .cpp file. Then, only one instance of the
pointer will be created by the compiler. I can understand that in the
templated case, one needs this declaration with the template code in
the .h file, but what stops the compiler from creating an instance of
the pointer for every inclusion of the header file?
The reason why there is only one CSingleton<T>::m_instance static
variable instantiated in the entire program is straightforward: C++
templates are not instantiated within translation units. Instead, each
template is instantiated within its own "instantiation unit" (which
roughly corresponds to a translation unit).
Therefore, all of CSingleton's instantiated methods and instantiated
static class objects are placed in a CSingleton instantiation unit -
thereby ensuring that only one CSingleton::m_instance is allocated
(per CSingleton instantiation) for the entire program. (?2.1/8 of the C
++ Standard describes instantiation units).
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