C++ 你如何传递成员函数指针？
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How do you pass a member function pointer?
提问by Matt Pascoe
I am trying to pass a member function within a class to a function that takes a member function class pointer. The problem I am having is that I am not sure how to properly do this within the class using the this pointer. Does anyone have suggestions?
我试图将类中的成员函数传递给采用成员函数类指针的函数。我遇到的问题是我不确定如何使用 this 指针在类中正确执行此操作。有没有人有建议？
Here is a copy of the class that is passing the member function:
这是传递成员函数的类的副本：
class testMenu : public MenuScreen{
public:
bool draw;
MenuButton<testMenu> x;
testMenu():MenuScreen("testMenu"){
x.SetButton(100,100,TEXT("buttonNormal.png"),TEXT("buttonHover.png"),TEXT("buttonPressed.png"),100,40,&this->test2);
draw = false;
}
void test2(){
draw = true;
}
};
The function x.SetButton(...) is contained in another class, where "object" is a template.
函数 x.SetButton(...) 包含在另一个类中，其中“对象”是一个模板。
void SetButton(int xPos, int yPos, LPCWSTR normalFilePath, LPCWSTR hoverFilePath, LPCWSTR pressedFilePath, int Width, int Height, void (object::*ButtonFunc)()) {
BUTTON::SetButton(xPos, yPos, normalFilePath, hoverFilePath, pressedFilePath, Width, Height);
this->ButtonFunc = &ButtonFunc;
}
If anyone has any advice on how I can properly send this function so that I can use it later.
如果有人对我如何正确发送此功能有任何建议，以便我以后可以使用它。
回答by Commodore Jaeger
To call a member function by pointer, you need two things: A pointer to the object and a pointer to the function. You need both in MenuButton::SetButton()
要通过指针调用成员函数，您需要两件事：指向对象的指针和指向函数的指针。你同时需要MenuButton::SetButton()
template <class object>
void MenuButton::SetButton(int xPos, int yPos, LPCWSTR normalFilePath,
LPCWSTR hoverFilePath, LPCWSTR pressedFilePath,
int Width, int Height, object *ButtonObj, void (object::*ButtonFunc)())
{
BUTTON::SetButton(xPos, yPos, normalFilePath, hoverFilePath, pressedFilePath, Width, Height);
this->ButtonObj = ButtonObj;
this->ButtonFunc = ButtonFunc;
}
Then you can invoke the function using both pointers:
然后您可以使用两个指针调用该函数：
((ButtonObj)->*(ButtonFunc))();
Don't forget to pass the pointer to your object to MenuButton::SetButton():
不要忘记将指向您的对象的指针传递给MenuButton::SetButton()：
testMenu::testMenu()
:MenuScreen("testMenu")
{
x.SetButton(100,100,TEXT("buttonNormal.png"), TEXT("buttonHover.png"),
TEXT("buttonPressed.png"), 100, 40, this, test2);
draw = false;
}
回答by Matt Cruikshank
I'd strongly recommend boost::bindand boost::functionfor anything like this.
我强烈推荐boost::bind，并boost::function为这样的事。
See Pass and call a member function (boost::bind / boost::function?)
回答by Yuanlong Li
I know this is a quite old topic. But there is an elegant way to handle this with c++11
我知道这是一个相当古老的话题。但是有一种优雅的方法可以用 c++11 来处理这个问题
#include <functional>
declare your function pointer like this
像这样声明你的函数指针
typedef std::function<int(int,int) > Max;
declare your the function your pass this thing into
声明你的函数，你将这个东西传递给
void SetHandler(Max Handler);
suppose you pass a normal function to it you can use it like normal
假设您向它传递了一个普通函数，您可以像平常一样使用它
SetHandler(&some function);
suppose you have a member function
假设你有一个成员函数
class test{
public:
int GetMax(int a, int b);
...
}
in your code you can pass it using std::placeholderslike this
在您的代码中，您可以std::placeholders像这样传递它
test t;
Max Handler = std::bind(&test::GetMax,&t,std::placeholders::_1,std::placeholders::_2);
some object.SetHandler(Handler);
回答by GKelly
Would you not be better served to use standard OO. Define a contract (virtual class) and implement that in your own class, then just pass a reference to your own class and let the receiver call the contract function.
使用标准 OO 会不会更好？定义一个合约（虚拟类）并在你自己的类中实现它，然后只需传递一个对你自己类的引用，让接收者调用合约函数。
Using your example (I've renamed the 'test2' method to 'buttonAction'):
使用您的示例（我已将“test2”方法重命名为“buttonAction”）：
class ButtonContract
{
public:
virtual void buttonAction();
}
class testMenu : public MenuScreen, public virtual ButtonContract
{
public:
bool draw;
MenuButton<testMenu> x;
testMenu():MenuScreen("testMenu")
{
x.SetButton(100,100,TEXT("buttonNormal.png"),
TEXT("buttonHover.png"),
TEXT("buttonPressed.png"),
100, 40, &this);
draw = false;
}
//Implementation of the ButtonContract method!
void buttonAction()
{
draw = true;
}
};
In the receiver method, you store the reference to a ButtonContract, then when you want to perform the button's action just call the 'buttonAction' method of that stored ButtonContract object.
在接收器方法中，您存储对 ButtonContract 的引用，然后当您想要执行按钮的操作时，只需调用该存储的 ButtonContract 对象的“buttonAction”方法。
回答by Johannes Schaub - litb
Others have told you how to do it correctly. But I'm surprised no-one told you this code is actually dangerous:
其他人已经告诉你如何正确地做到这一点。但我很惊讶没有人告诉你这段代码实际上是危险的：
this->ButtonFunc = &ButtonFunc;
Since ButtonFunc is a parameter, it will go out of scope when the function returns. You are taking its address. You will get a value of type void (object::**ButtonFunc)()(pointer to a pointer to a member function) and assign it to this->ButtonFunc. At the time you would try to use this->ButtonFunc you would try to access the storage of the (now not existing anymore) local parameter, and your program would probably crash.
由于 ButtonFunc 是一个参数，当函数返回时它将超出范围。你正在取它的地址。您将获得一个类型值void (object::**ButtonFunc)()（指向成员函数指针的指针）并将其分配给 this->ButtonFunc。当您尝试使用 this->ButtonFunc 时，您将尝试访问（现在不再存在）本地参数的存储，并且您的程序可能会崩溃。
I agree with Commodore's solution. But you have to change his line to
我同意 Commodore 的解决方案。但是你得把他的台词改成
((ButtonObj)->*(ButtonFunc))();
since ButtonObj is a pointerto object.
因为 ButtonObj 是一个指向对象的指针。
回答by Parappa
In the rare case that you happen to be developing with Borland C++Builder and don't mind writing code specific to that development environment (that is, code that won't work with other C++ compilers), you can use the __closure keyword. I found a small article about C++Builder closures. They're intended primarily for use with Borland VCL.
如果您碰巧使用 Borland C++Builder 进行开发并且不介意编写特定于该开发环境的代码（即不能与其他 C++ 编译器一起使用的代码），您可以使用 __closure 关键字. 我发现了一篇关于 C++Builder 闭包的小文章。它们主要用于 Borland VCL。
