1.mainwindow.h
	static MainWindow* getInstance();
2.  mainwindow.cpp
 Q_GLOBAL_STATIC(MainWindow,mainwindow)
 static MainWindow* MainWindow::getInstance(){
	return mainwindow();	
}

这样就可以使用了;
有时间在看内部的实现方法;
Qt 源码

#define Q_GLOBAL_STATIC(TYPE, NAME)                                         \
    Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ())

#define Q_GLOBAL_STATIC_WITH_ARGS(TYPE, NAME, ARGS)                         \                                                                   
    static QGlobalStatic<TYPE,Q_QGS_ ## NAME::innerFunction,Q_QGS_ ## NAME::guard> NAME;

struct QGlobalStatic
{
    typedef T Type;

    bool isDestroyed() const { return guard.load() <= QtGlobalStatic::Destroyed; }
    bool exists() const { return guard.load() == QtGlobalStatic::Initialized; }
    operator Type *() { if (isDestroyed()) return 0; return innerFunction(); }
    Type *operator()() { if (isDestroyed()) return 0; return innerFunction(); }
    Type *operator->()
    {
      Q_ASSERT_X(!isDestroyed(), "Q_GLOBAL_STATIC", "The global static was used after being destroyed");
      return innerFunction();
    }
    Type &operator*()
    {
      Q_ASSERT_X(!isDestroyed(), "Q_GLOBAL_STATIC", "The global static was used after being destroyed");
      return *innerFunction();
    }
};

#define Q_GLOBAL_STATIC_INTERNAL(ARGS)                          \
    Q_GLOBAL_STATIC_INTERNAL_DECORATION Type *innerFunction()   \
    {                                                           \
        struct HolderBase {                                     \
            ~HolderBase() Q_DECL_NOTHROW                        \
            { if (guard.load() == QtGlobalStatic::Initialized)  \
                  guard.store(QtGlobalStatic::Destroyed); }     \
        };                                                      \
        static struct Holder : public HolderBase {              \
            Type value;                                         \
            Holder()                                            \
                Q_DECL_NOEXCEPT_EXPR(noexcept(Type ARGS))       \
                : value ARGS                                    \
            { guard.store(QtGlobalStatic::Initialized); }       \
        } holder;                                               \
        return &holder.value;                                   \
    }

翻译完

static QGlobalStatic<MainWindow,Q_QGS_mainwindow::innerFunction,Q_QGS_mainwindow::guard> mainwindow;

typedef Type TYPE
所以 使用mainwindow()
Type *operator()() { if (isDestroyed()) return 0; return innerFunction(); }
返回一个Type 对象的指针；

innerFunction() 内部利用静态内部类构造对象 并进行析构；