Question

The source code of singleton of boost is there ,I don’t understand two notations in the source file below:

// ***include this to provoke instantiation at pre-execution time***
static void use(T const &) {};

BOOST_DLLEXPORT static T & get_instance() {
static detail::singleton_wrapper< T > t;
***// refer to instance, causing it to be instantiated (and
// initialized at startup on working compilers)***
BOOST_ASSERT(! detail::singleton_wrapper< T >::m_is_destroyed);
use(instance);
return static_cast<T &>(t);
}

Question is: How could this code force initialization of singleton in c++ before main()?What do these two notation mean?

Answer 1

It can’t. It’s this line that does:

template<class T>
BOOST_DLLEXPORT T & singleton< T >::instance = singleton< T >::get_instance();

It creates a static object that is initialized by a call to get_instance. Since it’s a class-static object, it’s initialized before main.