无法调用结构内部的成员函数指针

问题描述

我一直在绞尽脑汁，在声明，定义和最终调用程序内部的成员函数指针时正确使用语法。

I've been racking my brain over getting the syntax right on declaring, defining and finally calling a member function pointer inside my program.

我正在编写一个Xlib的窗口管理器，并试图使用户能够在 Keybind s向量中定义所有键绑定。 Keybind 结构包含更多成员变量，为简洁起见，在此省略了它们。

I'm writing a window manager with Xlib, and am trying to enable the user to define all key bindings in a vector of Keybinds. The Keybind struct contains more member variables, which I have left out here for the sake of brevity.

这是什么我到目前为止为止。

Here's what I've got so far.

Keybind ，一个包含成员变量的结构func ，它指向 MyClass 成员函数。

Keybind, a struct containing a member variable, func, that points to a MyClass member function.

struct MyBind {
    MyBind(void (MyClass::*_func)(const XKeyEvent&))
    : func(_func) {}

    void (MyClass::*func)(const XKeyEvent&);
}

向量的声明和填充包含用户定义的 Keybind s。

Declaration and populating of a vector that holds user-defined Keybinds.

// in my_class.hh
std::vector<MyBind*> my_binds_;

// in my_class.cc, constructor
my_binds_.push_back(new MyBind( &MyClass::do_this ));
my_binds_.push_back(new MyBind( &MyClass::do_that ));

此时，所有内容都会编译并运行。

At this point, everything compiles and runs.

现在，当我尝试通过遍历 my_binds _ 向量委派工作时，事情出了问题。值得注意的是，为了清楚起见，我省略了错误处理和其他成员变量访问。

Now, when I try to delegate work by iterating over the my_binds_ vector, things go wrong. It is worth noting that I've left out error handling and other member variable accesses for clarity.

void
MyClass::handle_input(const XKeyEvent& e)
{
    for (const MyBind* my_bind: my_binds_) {
        (my_bind->*func)(e); // erroneous line
    }
}

此，但编译失败，并指出 error：'func在此范围内未声明'（ g ++ ，与 clang ++ 类似的错误）。

This should be the correct syntax, but it fails to compile, stating error: ‘func’ was not declared in this scope (g++, similar error from clang++).

这对我来说很奇怪，因为用 auto test = keybind-&f; func;替换了错误的代码行； 可以编译。

This is weird to me, as replacing the erroneous line of code with auto test = keybind->func; does compile.

我在做什么错？有没有更好的方法来处理用户键绑定定义？谢谢！

What am I doing wrong? Is there a better way to handle user key bind definitions? Thanks!

推荐答案

最好使用并完全忘记了原始成员函数指针。它们只会给您带来痛苦：）

It would be best to use std::function and forget about raw member-function pointers altogether. They will only bring you pain :)

代码的问题在于，您只有一个指向方法的指针，而没有对象。您的绑定结构还应该存储一个对象指针，以在以下位置调用该方法：

The problem with you code is that you only have a pointer to a method but no object. Your bind struct should also store an object pointer to call the method on:

struct MyBind {
    MyBind(MyClass *obj, void (MyClass::*_func)(const XKeyEvent&))
    : obj(obj), func(_func) {}

    MyClass *obj;
    void (MyClass::*func)(const XKeyEvent&);

    void operator()(const XKeyEvent& event) const
    {
        (obj->*func)(event);
    }
}

然后像这样使用它：

void
MyClass::handle_input(const XKeyEvent& e)
{
    for (const MyBind* my_bind: my_binds_) {
        (*my_bind)();
    }
}

我在绑定结构中添加了一个调用运算符为了方便。请注意，-> * 运算符应用于该方法所属的对象。

I've added a call operator to the bind struct for convenience. Note that the ->* operator is applied to the object the method belongs to.

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