c++ late instantiation of global variable

Question

I have code where a global resource has to be set up by quite some code:

globalClass foo;  // global variable / object; give it a memory space to live

void doSomething( void )
{
  foo.bar();      // use the global foo object
}

int main( int argc, const char *argv[] )
{
  foo( argc );   // foo can only be instantiated in main as it's using
                 // information that's only available here

  doSomething(); // use the global foo object

  return 0;
}

As you can see, foo is of global scope - but to call its constructor I need some information that's only available inside of main.

How can I achieve that?

The only solution I could figure out is to make foo a pointer to globalClass - but that would result in a pointer dereferencing each time I'm using foo. This might create a performance problem when used in a tight loop...

PS: In the real program main and doSomething would live in different files. And it's of course guaranteed that foo won't be accessed before it's instantiated.

Answer 1

Here's a really terrible hack if you don't want the indirection and don't mind doing the cleanup yourself:

union ClassHolder
{
    globalClass x;
    char buf[sizeof globalClass];

    constexpr ClassHolder() noexcept : buf() { }
};

ClassHolder h;
globalClass & foo = h.x;

int main()
{
    new (static_cast<void *>(&h.x)) globalClass(arg1, arg2, arg3);

    // main program

    h.x.~globalClass();
}