///////////////////// my.cpp ///////////////
CMyWinApp theApp;void main()
{CWinApp* pApp = AfxGetApp();}////////////////////// my.h ////////////////class CMyWinApp : public CWinApp
{
public:
CMyWinApp::CMyWinApp() {
cout << "CMyWinApp Constructor \n";
}
CMyWinApp::~CMyWinApp() {
cout << "CMyWinApp Destructor \n";
}
};
////////////////// mfc.h ///////////////class CObject
{
public:
CObject::CObject() {
cout << "CObject Constructor \n";
}
CObject::~CObject() {
cout << "CObject Destructor \n";
}
};class CCmdTarget : public CObject
{
public:
CCmdTarget::CCmdTarget() {
cout << "CCmdTarget Constructor \n";
}
CCmdTarget::~CCmdTarget() {
cout << "CCmdTarget Destructor \n";
}
};class CWinThread : public CCmdTarget
{
public:
CWinThread::CWinThread() {
cout << "CWinThread Constructor \n";
}
CWinThread::~CWinThread() {
cout << "CWinThread Destructor \n";
}
};
class CWinApp : public CWinThread
{
public:
CWinApp* m_pCurrentWinApp;public:
CWinApp::CWinApp() {
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}
CWinApp::~CWinApp() {
cout << "CWinApp Destructor \n";
}
};
// global functionCWinApp* AfxGetApp();为了产生全局对象theApp,从基类开始构造,依次是
CObject Constructor
CCmdTarget Constructor
CWinThread Constructor
CWinApp Constructor
CMyWinApp Constructor
然后全局对象theApp才产生,那么在CWinApp的构造函数里:
CWinApp::CWinApp() {
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}这个this指向哪里(哪个对象),this应该只能指向对象吧, 应该不是CMyWinApp的对象,因为它的对象还没有产生(构造函数还没有执行).然后看main里的全局函数AfxGetApp()调用,它的定义是:CWinApp* AfxGetApp()
{
return theApp.m_pCurrentWinApp;
}当然,进入main前theApp已经产生了,它继承了CWinApp里的数据成员m_pCurrentWinApp,也就是theApp自己拷贝了一份,m_pCurrentWinApp在先前的CWinApp的构造函数里已经被赋值了,所以它拷贝的这份m_pCurrentWinApp里面也有那个this,接下来返回那个this给CWinApp* pApp我现在的疑惑是那个this到底指向何方? 以前没仔细看程序,一直认为是指向theApp这个全局对象,当然现在我也是这样认为的,但好象不太符合逻辑,还是我哪里理解错了?
CMyWinApp theApp;void main()
{CWinApp* pApp = AfxGetApp();}////////////////////// my.h ////////////////class CMyWinApp : public CWinApp
{
public:
CMyWinApp::CMyWinApp() {
cout << "CMyWinApp Constructor \n";
}
CMyWinApp::~CMyWinApp() {
cout << "CMyWinApp Destructor \n";
}
};
////////////////// mfc.h ///////////////class CObject
{
public:
CObject::CObject() {
cout << "CObject Constructor \n";
}
CObject::~CObject() {
cout << "CObject Destructor \n";
}
};class CCmdTarget : public CObject
{
public:
CCmdTarget::CCmdTarget() {
cout << "CCmdTarget Constructor \n";
}
CCmdTarget::~CCmdTarget() {
cout << "CCmdTarget Destructor \n";
}
};class CWinThread : public CCmdTarget
{
public:
CWinThread::CWinThread() {
cout << "CWinThread Constructor \n";
}
CWinThread::~CWinThread() {
cout << "CWinThread Destructor \n";
}
};
class CWinApp : public CWinThread
{
public:
CWinApp* m_pCurrentWinApp;public:
CWinApp::CWinApp() {
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}
CWinApp::~CWinApp() {
cout << "CWinApp Destructor \n";
}
};
// global functionCWinApp* AfxGetApp();为了产生全局对象theApp,从基类开始构造,依次是
CObject Constructor
CCmdTarget Constructor
CWinThread Constructor
CWinApp Constructor
CMyWinApp Constructor
然后全局对象theApp才产生,那么在CWinApp的构造函数里:
CWinApp::CWinApp() {
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}这个this指向哪里(哪个对象),this应该只能指向对象吧, 应该不是CMyWinApp的对象,因为它的对象还没有产生(构造函数还没有执行).然后看main里的全局函数AfxGetApp()调用,它的定义是:CWinApp* AfxGetApp()
{
return theApp.m_pCurrentWinApp;
}当然,进入main前theApp已经产生了,它继承了CWinApp里的数据成员m_pCurrentWinApp,也就是theApp自己拷贝了一份,m_pCurrentWinApp在先前的CWinApp的构造函数里已经被赋值了,所以它拷贝的这份m_pCurrentWinApp里面也有那个this,接下来返回那个this给CWinApp* pApp我现在的疑惑是那个this到底指向何方? 以前没仔细看程序,一直认为是指向theApp这个全局对象,当然现在我也是这样认为的,但好象不太符合逻辑,还是我哪里理解错了?
但使用 AfxGetApp() 时即使存在多个 CxApp 也只会返回 theApp 对象的 this 指针
CWinApp::CWinApp() {
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}
this就是指向CWinApp类的指针
cout << "CWinApp Constructor \n";
m_pCurrentWinApp = this;
}
this指向CWinApp对象。this指针作为隐含参数传递给构造函数,所以构造函数可以访问this指针。构造函数执行时对象还没有产生(构造函数还没有执行)这句话是错误的。
你在哪个对象或类中使用this,它就是哪种类型的指针,如果当前有多个实例,则指向当前实例。类也是有实例的,在软件运行过程中可能产生某个类的数据类型的多个变量实例,而this就等于每个实例变量自己的地址。
明白否?
class A;
A(){
//这个时候已经建立对象
//可以在这个时候对成员变量赋值就是证据};
是的,我以前把构造函数和其他成员函数割裂开了,认为只有构造函数的右大括号结束后对象才产生,经过各位的提醒我也想明白了,既然在构造函数里可以对成员变量赋值,那么具体对象的this确实是隐含地传给了构造函数,原因很简单,因为每个对象都有成员变量的一份拷贝,而构造函数只有一个,如果没有this,它便无法确定向哪个对象的成员变量赋值.
是这样理解吧