成员的const引用是否安全 [英] Are const references to members safe

问题描述

如果我使用const引用另一个成员， 此引用是否有可能无效?

If I use a const reference to another member, is it possible that this reference gets invalidated?

class Class {
public:
    const int &x{y};
private:
    int y;
};

例如，当我在矢量中使用此类的实例时 在push_back之后增加其容量. 根据标准，如果满足以下条件，则所有迭代器和引用均无效 载体必须增加其容量.此后引用仍然有效吗?

For example when I use instances of this class in a vector which increases its capacity after a push_back. According to the standard all iterators and references are invalidated if a vector has to increase its capacity. Is the reference still valid after that?

推荐答案

当前不安全，因为当您复制Class的实例时，x会引用复制对象的y，而不是其对象拥有y.您可以通过运行以下代码来查看此内容:

This is currently not safe, as when you copy an instance of Class, x will reference the y of the copied object, not its own y. You can see this by running the following code:

int main()
{
    Class a{};
    std::vector<Class> vec;
    vec.push_back(a);

    //these lines print the same address
    std::cout << &(a.x) << std::endl;
    std::cout << &(vec[0].x) << std::endl;
}

您可以通过编写自己的副本构造函数和赋值函数来正确初始化x来解决此问题:

You can fix this by writing your own copy constructor and assignment functions to correctly initialize x:

Class (const Class& rhs) : x{y}, y{rhs.y} {}

这是安全的，因为x和y只会与您的对象一起被销毁.对std::vector的引用无效意味着对矢量元素的引用:

This is safe, becausex and y will only be destroyed along with your object. Invalidation of references for std::vector means references to the vector elements:

Class c;
std::vector<Class> vec;
vec.push_back(c);

Class& cr = vec[0];
//other operations on vec
std::cout << c.x; //fine, that reference is internal to the class
std::cout << cr.x; //cr could have been invalidated

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