Global and singleton

虽然两种都能够实现execute once、single object，但是相比于global，singleton有着优势:

1、

singleton: construct on first use

global: initialisation order fiasco

stackoverflow extern vs Singleton class

A

Global variables suffer from the initialisation order fiasco. Globals in separate translation units are initialised in an unspecified order, so you get undefined behaviour if one refers to another in its constructor.

Static variables in a function scope are initialised the first time the function is called, which (more or less) solves the initialisation order problem.

However, more subtle problems remain; in particular, it is still possible to access the object after it has been destroyed, and there is some overhead to ensure thread-safe initialisation (and, in older compilers, initialisation might not be thread-safe at all). There is no completely safe way to manage globally accessible objects of non-trivial types, and my advice would be to avoid them altogether if possible.

Example: MFC theApp

MFC的theApp是全局唯一的，它的声明与定义如下：

CMFCDemoApp.h

class CMFCDemoApp : public CWinApp
{
public:
    CMFCDemoApp();

// 重写
public:
    virtual BOOL InitInstance();

// 实现

    DECLARE_MESSAGE_MAP()
};

extern CMFCDemoApp theApp;

CMFCDemoApp.cpp

// 唯一的 CMFCHSTradeApiDemoApp 对象

CMFCDemoApp theApp;

TODO

1、ogre3d Singleton vs. extern/global variable