如果我可以使用指向类中变量的普通指针，那么类成员指针的意义何在？

发布于 2024-09-26 23:24:27 字数 382 浏览 1 评论 0原文

拿这段代码来说：

struct mystruct
{
    int var;

    mystruct() : var(0) {}
};

int main()
{
    mystruct ins;

    int* p = &ins.var;

    *p = 1;
}

那么，类成员指针的一些具体的、非常好的使用示例有哪些呢？

int    X::*p = &X::data; /* p contains offset */
X      object;
X      *objptr = new X;

int    i = object.*p;
int    j = objptr->*p;

原文

Take this code:

struct mystruct
{
    int var;

    mystruct() : var(0) {}
};

int main()
{
    mystruct ins;

    int* p = &ins.var;

    *p = 1;
}

So what are some concrete really good examples of uses for the class member pointer?

int    X::*p = &X::data; /* p contains offset */
X      object;
X      *objptr = new X;

int    i = object.*p;
int    j = objptr->*p;

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魔法少女 2024-10-03 23:24:27

看来您的问题是关于指向数据成员类型的指针。在 C++ 中，还有指向成员函数的指针类型。两者在某些抽象层面上有一些共同点，但在其他方面却有所不同。

指向数据成员类型的指针适用于该类的任何实例。普通指针总是指向类的特定实例的成员。指向数据成员类型的指针是从类对象开头开始的“运行时偏移”思想的更高级别实现。它是一个相对指针。这样，它与普通指针完全不同，普通指针是绝对指针。

为了通过示例来说明这一点，假设您有一个类数组，其中包含三个相同类型的成员 x、y 和 z

struct Point { int x, y, z; };

Point point_array[N];

并且您需要将整个数组中所有同名的成员设置为0，而不更改任何其他成员。下面是如何使用指向数据成员类型的指针来完成此操作

void zero_members(Point points[], int n, int Point::*m) {
  for (int i = 0; i < n; ++i)
    points[i].*m = 0;
}

。现在，通过使用此函数，您可以

zero_members(point_array, N, &Point::x);

将所有 x 设置为零。或者您可以

zero_members(point_array, N, &Point::y);

将所有 y 设置为零。您可以使用一个单个函数来完成所有这些操作，而且同样重要的是，成员选择是由运行时参数执行的（而不是编译时参数）时间一）。

你不能用普通的指针做这样的事情。事实上，你无法通过任何其他方式做到这一点。

It seems the your question is about pointers of pointer-to-data-member type. In C++ there are also pointers of pointer-to-member-function type. The two have something in common at some abstract level, but otherwise they are different.

Pointer of pointer-to-data-member type is applicable to any instance of the class. An ordinary pointer always points to a member of a specific instance of the class. Pointer of pointer-to-data-member type is a higher level implementation of the idea of "run-time offset" from the beginning of a class object. It is a relative pointer. In that way it is completely different thing from ordinary pointers, which are absolute pointers.

To illustrate that with an example, let's say you have an array of classes with three members of the same type x, y and z

struct Point { int x, y, z; };

Point point_array[N];

and you need to set all members with the same name to 0 in the entire array, without changing any other members. Here's how you can do it using a pointer of pointer-to-data-member type

void zero_members(Point points[], int n, int Point::*m) {
  for (int i = 0; i < n; ++i)
    points[i].*m = 0;
}

Now, by using this function you can do

zero_members(point_array, N, &Point::x);

to set all xs to zero. Or you can do

zero_members(point_array, N, &Point::y);

to set all ys to zero. You do can all this with a single function, and what's also important, the member selection is performed by a run-time parameter (as opposed to a compile-time one).

You cannot do something like this with ordinary pointers. In fact, you can't do it in any other way.

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