return 指向在堆上创建的对象成员的指针

Question

所以如果我有 class

class Item {
 public:
  Item(int key, int value):key(key), value(value) {}
  int GetValue() {
    return value;
  }
 private:
  int key;
  int value;
}

如果我在堆上创建一个 Item 对象：

Item *item = new Item(1, 2);

是否可以获取指向该对象值成员的指针？

比如我有另一个方法class

int* GetValue(int k) {
  Item* item = ...;  // It finds the pointer to an Item object with key k that was created before.
  // Is there anyway I can return the pointer to the value member of this item?
 // Something like:
 return &(item->GetValue());  // This won't compile because item->GetValue() is a r-value. Is there any alternative way to walk around?
}

Answer 1

尝试将这行代码添加到您的代码中。

template <class Tag, typename Tag::type Value>
struct Getter
{
    friend typename Tag::type get(Tag) {
        return Value;
    }
};

struct Tag_item {
    typedef int Item::*type;
    friend type get(Tag_item);
};

int* GetValue(int k) {
    static Item item(13, 14);
    // std::cout << item.GetValue() << std::endl; // This line to test output.
    return &(item.*get(Tag_item())); // If pItem is pointer then pItem->*get(Tag_item())
}
template struct Getter<Tag_item, &Item::value>;

在 http://coliru.stacked-crooked.com/a/3be5b80e848650c5

查看演示

归功于http://bloglitb.blogspot.com/2011/12/access-to-private-members-safer.html by Johannes Schaub - litb

return 指向在堆上创建的对象成员的指针

return a pointer to a member of an object that is created on the heap

c++

pointers

object

rvalue