How singleton in boost implement all singletons are initialized before main is called?

问题

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?

回答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.

回答2:

I think the first comment actually refers to the line above,

BOOST_DLLEXPORT static T & instance;

which constructs a static instance of T. Statics are initialized before main is started, that's just a C++ rule.

回答3:

You're just showing part of the code, which may explain why you don't understand.

Before a program is executed, it must first be loaded into memory. At that time, if a static or global scope variable is found, it is initialized with either the value specified or the default compiler.

Then if you specify a static instance of class, it's constructor will be called even before main! That what boost uses to provide this functionality.

来源：https://stackoverflow.com/questions/8488108/how-singleton-in-boost-implement-all-singletons-are-initialized-before-main-is-c