将成员函数作为参数传递给构造函数 [英] Passing a member function as an argument to a constructor

问题描述

我有一个按钮类。我想让按钮类的构造函数接受当按下按钮时它将调用的函数。这很容易，如果按钮类只是从一个类中取一个函数，但按钮的构造函数接受一个函数的目的是，无论什么类的按钮被创建，它将能够存储一个指针的函数

I have a button class. I want the button class's constructor to take the function it will call when the button is pressed. This would be easy if the button class was only taking a function from one class, but the purpose of the button's constructor taking a function is that no matter what class the button is created in it will be able to store a pointer to a function in the class it's created in.

示例：

struct Button {
    Button(void (*functionPtr)()) {
        // Store the function pointer to be called later
    }
};

struct SomeClass {
    void LoadFile();

    SomeClass() {
        Button* temp1 = new Button(&LoadFile); // ???
    }
};

struct AnotherClass {
    void SaveFile();

    SomeClass() {
        Button* temp2 = new Button(&SaveFile); // ???
    }
};

如何使这项工作？

推荐答案

尽管看起来很相似，指针函数和指针成员函数实际上是完全不同的野兽。

A pointer-to-function and a pointer-to-member-function, despite seeming pretty similar, are actually entirely different beasts.

void（* functionPtr）（）是一个指向不带参数并返回void的函数的指针。 & AnotherClass :: SaveFile 是指向 AnotherClass 的成员函数的指针。它的类型是 void（AnotherClass :: *）（）。请注意，类名是类型的一部分，因此您不能简单地将指针到成员函数存储到任意类。此外，要调用指针到成员函数，你需要一个实例指针 - 你必须以某种方式存储，但那些也将有不同的类型！

void (*functionPtr)() is a pointer to a function that takes no arguments and returns void. &AnotherClass::SaveFile is a pointer to a member function of AnotherClass... its type is void (AnotherClass::*)(). Notice that the class name is part of the type, so you can't simply store a pointer-to-member-function to an arbitrary class. Furthermore, to call a pointer-to-member-function, you need an instance pointer - you'd have to store that somehow, but those would have different types too!

在C ++ 11中可以做的是使用type-erasure：

What you could do instead in C++11 is use type-erasure:

std::function<void()> callback;

并为其指定一个任意的可调用：

And assign an arbitrary callable to it:

template <typename F>
Button(F cb) 
: callback(cb)
{ }

然后，您可以使用 std :: bind 创建按钮：

And then you could create a button using std::bind:

Button* temp1 = new Button(std::bind(&OtherClass::LoadFile, this));
Button* temp2 = new Button(std::bind(&AnotherClass::SaveFile, this));

现在 temp1-> callback（）实际上会调用 OtherClass 的实例调用 LoadFile（）。这是你想要的。而且，我们仍然可以使用自由函数：

Now temp1->callback() will actually call LoadFile() on the instance of OtherClass that it was constructed with. Which is what you wanted. And, we can still use free functions:

void whatever();
Button* temp3 = new Button(whatever);

插入关于使用原始指针和 new 并偏好 unique_ptr 。

Insert the usual caveats about using raw pointers and new and preferring unique_ptr.

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