指针和双向链表

Question

I am currently on chapter 17 of "Programming: Principles and Practice using C++", and I came across a code on doubly-linked list. Questions have been asked on this set of codes before for (eg Member access in a doubly-linked list ), however I have a question that wasn't asked/answered.

The code basically arranges "Freya" as a predecessor of "Odin", and makes "Odin" a predecessor of "Thor", and the other way round as successors. The code is as follows:

struct Link {
string value;
Link* prev;
Link* succ;
Link(const string& v, Link* p = nullptr, Link* s = nullptr)
    : value(v), prev(p), succ(s) {}
};

int main()
{
    Link* norse_gods = new Link{ "Thor", nullptr, nullptr };
    norse_gods =       new Link{ "Odin", nullptr, norse_gods };

    norse_gods->succ->prev = norse_gods;
    norse_gods =       new Link{ "Freya", nullptr, norse_gods };

    norse_gods->succ->prev = norse_gods;

}

What I would like to know is that why the code:

norse_gods =       new Link{ "Odin", nullptr, norse_gods }

is able to point to the old address: new Link{ "Thor", nullptr, nullptr } , and making Thor the successor of Odin since norse_god should already be pointing to the new address: new Link{ "Odin", nullptr, norse_gods } ? Is there some order or evaluation or concept that I am missing out?

Answer 1

The evaluation is first done on the right side and then operator= kicks in and does the assignment. This is caused by the priority of operator= . See C++ Operator Precedence .

At the first line of main , you are creating a Link with name "Thor" and pointing norse_gods to it.

At the second line, you are creating a Link with name "Odin" and a successor norse_god , which is still pointing at Thor at the moment of construction.

After that, norse_gods is updated to point to "Odin".

This behavior is normal for C-like languages.

Answer 2

The expression on the right-hand side is evaluated first. This evaluates taking the old value of norse_gods . Once this evaluation is complete, the assignment is done, and the reference to the newly created object is assigned to norse_gods .