c++ - 为什么在ctor中不允许访问成员对象的成员？

Question

边缘类:

class Edge {
    int dist = 0;
    std::pair<Node, Node> ends;
public:
    Edge() = default;
    explicit Edge(const int idist) : dist(idist) { }
    explicit Edge(const int idist, Node& end1, Node& end2) : dist(idist) {
        ends.first = end1;
        ends.second = end2;
    }
    ~Edge() = default;
};

在ctor explicit Edge(const int idist, Node& end1, Node& end2)中，为什么不允许使用该语法？:

explicit Edge(const int idist, Node& end1, Node& end2) : dist(idist), ends.first(end1), ends.second(end2) { }

Answer 1

只是不允许这样做。作为member initializer list的语法，

class-or-identifier ( expression-list(optional) ) (1) 
class-or-identifier brace-init-list   (2) (since C++11)

尽管 ends.first和 ends.second不引用类或标识符，但它们是表达式。您必须整体初始化 ends，例如

explicit Edge(const int idist, Node& end1, Node& end2) : dist(idist), ends(end1, end2) { }