我可以在C ++中创建匿名类并捕获外部变量，如在Java？ [英] Can I create anonymous classes in C++ and capture the outside variables like in Java?

问题描述

在Java中，当我需要一个回调函数时，我必须实现一个匿名类。在匿名类中，如果 final ，我可以访问外部变量。

In Java, when I need a callback function, I have to implement an anonymous class. Inside the anonymous class, I can access the outside variables if they're final.

在C ++中的同样的事情。我理解，C ++ lambda的工作更好，但有时我需要传递在许多函数中与匿名类，我只需要传递一个实例。

Now I'm doing the same thing in C++. I understand that C++ lambda works better but sometimes I need to pass in many functions where with anonymous classes, I only need to pass in one instance.

我试过下面的例子。它与GCC 4.3.4一起工作。

I tried the following example. It works with GCC 4.3.4.

class IA {
public:
  virtual int f(int x) = 0;  
};

int main() {
    class : public IA {
        int f(int x) { return x + 1; }
    } a;
    doFancyWork(&a);
    return 0;
}

是否可以捕获这样的外部变量？

Is it possible to capture the outside variables like this?

int main() {
    int y = 100; // mark y as final if possible
    class : public IA {
        int f(int x) { return x + y; }
    } a;
    return 0;
}

UPDATE：

第二个例子将不编译。错误在这里，

The second example won't compile. The errors are here,

prog.cpp: In member function ‘virtual int main()::<anonymous class>::f(int)’:
prog.cpp:9: error: use of ‘auto’ variable from containing function
prog.cpp:7: error:   ‘int y’ declared here
prog.cpp: In function ‘int main()’:
prog.cpp:7: warning: unused variable ‘y’

更新：

我刚刚意识到了更多的问题：

I just realized a few more problems in doing this:

• 我无法写入构造函数，因为类没有名称


• 初始值设置列表不允许继承。

我认为我不得不改变匿名类。

I think I have to move away from anonymous classes.

推荐答案

没有办法自动捕获这些变量，但可以使用替代方法。这是如果你想捕获通过引用：

There is no way to automatically capture those variables, but you can use an alternative approach. This is if you want to capture by reference:

int main() {
    int y = 100; // mark y as final if possible
    class IB : public IA {
    public:
      IB(int& y) : _y(y) {}
      int f(int x) { return x + _y; }
    private:
      int& _y;
    } a (y);
    return 0;
}

如果要按值捕捉，只需更改

If you want to capture by value, just change int& into int.

无论如何，你可以考虑使用

Anyway, you may consider using a tuple of lambdas as a "multi-callback" object if that is what bothers you about individual lambdas. You would still have everything packed in one object and capturing would be done for free.

只是作为一个例子：

auto callbacks = make_tuple(
    [] (int x) { cout << x << endl; },
    [&] () { cout << y << endl; }, // y is captured by reference
    [=] (int x) { cout << x + y << endl; }, // y is captured by value
    // other lambdas here, if you want...
    );

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