std::vector abort problem

I'm having diffuculty with the example below, the last line is producing an "abort has been called" error. I don't see why this should be.

I'm using (*abc).def instead of abc->def for clarity in this case.

#include <iostream>
#include <string>
#include <vector>

class branch
{
    public:
        unsigned short n;
        std::vector<branch> branches;

        branch left()
        {
            return branches.at(0);
        }
};

void main()
{
    branch trunk = branch();
        trunk.n = 0;
        branch b1, b2;
        b1.n = 0;
        b2.n = 5;
        b1.branches.push_back(b2);
        trunk.branches.push_back(b1);

    branch* focus1 = &(trunk.branches.at(0));
    branch* focus3 = &(trunk.left());

    std::cout<<trunk.left().branches.at(0).n<<std:开发者_开发技巧:endl; // ok
    std::cout<<(*focus1).branches.at(0).n<<std::endl; // ok
    std::cout<<(*focus1).left().n<<std::endl; // ok
    std::cout<<(*focus3).branches.at(0).n<<std::endl; // problem
}

The problem with this code is that trunk.left() returns a copy of the branch, not a reference to the branch. Consequently, your focus3 pointer is pointing at a temporary object that's immediately going to be cleaned up after that line of code finishes executing. Consequently, when you try dereferencing focus3 on the last line, you're following a pointer to garbage data, which causes the crash.

To fix this, either have left return a reference to the branch, or make focus3 a const reference, which extends the lifetime of the temporary to the lifetime of the reference.