第15章《乐趣》Page305~311, 代码精简以后，讨论一下引用含义的问题

将Page305~311的代码精简了一下，讨论一下引用含义的问题，精简之后的代码如下：

#include <iostream>
#include <SDL2/SDL.h>using namespace std;namespace sdl2
{char const* last_error()
{return SDL_GetError();
}struct Initiator
{
private:Initiator()//默认构造: _init_result(-1){}
public:static Initiator& Instance(){static Initiator Instance;return Instance;}~Initiator(){if(_init_result == 0){SDL_Quit();}}void GetVersion(Uint8& major, Uint8& minor, Uint8& patch){SDL_version ver;SDL_GetVersion(&ver);major = ver.major;minor = ver.minor;patch = ver.patch;}bool Init(Uint32 flags = SDL_INIT_EVERYTHING){_init_result = SDL_Init(flags);return 0 == _init_result;}//    bool operator bool()explicit operator bool() const{return _init_result == 0;}private:int _init_result;
};//Initiatorstruct WindowPosition
{WindowPosition() //默认构造，_x,_y都使用系统默认位置//SDL_WINDOWPOS_UNDEFINED会将窗口放到中间: _x(SDL_WINDOWPOS_UNDEFINED), _y(SDL_WINDOWPOS_UNDEFINED){}//设置为居中，注意返回当前对象的引用WindowPosition& Centered(bool x_centered = true, bool y_centered = true){if(x_centered)_x = SDL_WINDOWPOS_CENTERED;if(y_centered)_y = SDL_WINDOWPOS_CENTERED;}int _x, _y;
};struct WindowFlags
{WindowFlags() //默认构造，用于构建一个没有指定任何特性的普通窗口: _flags(0){}Uint32 _flags;
};struct Window
{
public:Window(char const* title, WindowPosition const& win_position, int w, int h, WindowFlags const& win_flags){cout << &win_position << endl;_window = SDL_CreateWindow(title, win_position._x, win_position._y, w, h, win_flags._flags);}~Window(){if(_window) //_window不是空指针{SDL_DestroyWindow(_window);}}SDL_Window* _window;
};}int main(int argc, char* argv[])
{sdl2::Initiator::Instance().Init(SDL_INIT_VIDEO| SDL_INIT_AUDIO| SDL_INIT_EVENTS| SDL_INIT_TIMER);if(!sdl2::Initiator::Instance())//重载转换符{cerr << "初始化就出错，没得玩了！"<< sdl2::last_error() << endl;return -1;}//创建并居中显示宽640高480的游戏窗口sdl2::Window wnd("hello sdl", sdl2::WindowPosition().Centered(true, true), 640, 480//使用空的特性标志, sdl2::WindowFlags());SDL_Delay(5000);return 0;
}

可以看到代码第72和99行，使用了引用符号&，打两个断点，调试一下，当程序运行到75行时，停了下来，如下图：