创建指向非静态类成员函数的类成员指针函数变量 [英] Creating a class member pointer function variable that points to a non-static class member function

问题描述

目标是让成员变量 _AddValue 在类初始化时和第一次初始化后指向 CreateFirstValue 函数调用 AddValue ，将来对其进行的所有调用都会调用 CreateAnotherValue 。

The goal is to have the member variable _AddValue point to the CreateFirstValue function upon class initialization and after the first invocation of AddValue, all future calls to it will invoke CreateAnotherValue.

以前，我只有一个 AddValue 函数，并通过条件检查来确定要调用的函数。但是，我觉得这种实现是有缺陷的，因为 if 检查每次都会发生，并且似乎函数指针在这里会很有用。

Previously, I just had a single AddValue function with a conditional check to determine which function to call. However, I feel like that implementation is flawed because that if check will occur every time and it seems like a function pointer would be beneficial here.

示例：

class Foo
{
 private:
  int _value;
  void (*_AddValue)(int value); // Pointer to function member variable

  void CreateFirstValue(int value)
  {
    _value = value;
    _AddValue = &CreateAnotherValue;
  }

  void CreateAnotherValue(int value)
  {
    // This function will create values differently.
    _value = ...;
  }

 public:
  // Constructor
  Foo()
   : _value(0), _AddValue(CreateFirstValue)
  {
  }

  AddValue(int value) // This function is called by the user.
  {
    _AddValue(value);
  }
};

上面的代码不是实际的代码，只是我要完成的示例。

The code above is not the actual code, just an example of what I'm trying to accomplish.

现在我遇到了一个错误： void（BTree ::）（int）类型的参数与void（*）（ int）

right now I'm getting an error: argument of type void (BTree::)(int) does not match void (*)(int)

推荐答案

&CreateAnotherValue

此语法无效。要创建指向成员的指针，您甚至必须从其他成员内部为类命名。尝试

This syntax is not valid. To create a pointer-to-member, you have to name the class, even from inside other members. Try

&Foo::CreateAnotherValue

在这种情况下，您正在说的是 合格 非静态成员函数的地址，该地址是允许的，并且可以防止有关地址的错误不合格成员函数。

In this case you are talking the address of a qualified non-static member function, which is allowed and prevents the error about address of unqualified member function.

当然，您需要一个适当类型的变量来存储指向成员的指针，请参见Bo的答案为正确的声明。需要调用它时，您将需要使用指向成员取消引用的指针运算符（。* 或->> ），这样说

Of course, you then need an appropriately typed variable to store the pointer-to-member in, see Bo's answer for the correct declaration. When it comes time to call it, you will need to use the pointer-to-member-dereference operator (either .* or ->*), so say

(this->*_AddValue)(whatever);

同一规则适用于数据，如果您说& Foo :: _value ，您将得到一个指向 int Foo :: * 类型的成员的指针。但是在数据情况下，也可以接受非限定名称，但行为却大不相同。 & _value 给出普通指针，键入 int * ，这是特定此实例中的> _value 成员变量。

The same rule applies to data, if you say &Foo::_value, you get a pointer-to-member of type int Foo::*. But in the data case, the unqualified name is also accepted, but with very different behavior. &_value gives a normal pointer, type int*, which is the address of the specific _value member variable inside the this instance.

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