Vfrdtyky

#include <iostream>

#include <vector>

#include <string>





struct A

{

    std::string a;

    std::vector<A> avector;

};



typedef std::vector<A> Avector;



A& func(A& x)

{

   A& ret = x.avector[0];

   return ret;

}



int main()

{

   A genesis = { "Parent", std::vector<A>() };

   A l1 = { "Child1", std::vector<A>() };

   A l2 = { "Child2", std::vector<A>() };



   genesis.avector.push_back(l1);

   genesis.avector.push_back(l2);



   std::cout << "l1: " << l1.a << std::endl; //shows "Child1"

   std::cout << "l2: " << l2.a << std::endl; //shows "Child2"



   A& lx = func(genesis);

   lx.a = "Childx";



   std::cout << "l1: " << l1.a << std::endl; //!!still shows "Child1"



   return 1;

}

So, basically what i want is to have a single copy of data overall i.e., Genesis object and two more objects l1 and l2 as objecst of Genesis.avector

However I'm unable to modify this later as every time I end up modifying the copies but not the actual data under Genesis object.

Thanks for your help!

In your code:

A genesis = { "Parent", std::vector<A>() };

A l1 = { "Child1", std::vector<A>() };

A l2 = { "Child2", std::vector<A>() };



genesis .avector.push_back(l1);

genesis .avector.push_back(l2);

You have 5 A instances. The 3 that you declare and the 2 copies you have in your vector.

So the more correct (but possible buggy) way to do it would be:

A genesis = { "Parent", std::vector<A>() };

genesis .avector.emplace_back("Child1", std::vector<A>());

genesis .avector.emplace_back("Child2", std::vector<A>());



A& l1 = genesis.avector[0];

A& l2 = genesis.avector[1];

Now you only have 3 instances. l1 and l2 are references to the items in the vector, so changes to them will be refleced in the vector as well.

I said this is possibly buggy. When you alter the vector (adding something else), the vector might have to reallocate so any references you have will be invalid and the result is undefined behavior.

If you do need to modify the vector, I would do vector<unique_ptr<A>>. Then A& l1 = *genesis.avector[0] and this will remain valid until the item get's removed from the vector.

You can also try std::reference_wrapper instead of unique_ptr if you want the objects to live on the stack.
I would try to avoid this as it's more obvious that something get destroyed when you erase from a vector than when a local variable goes out of scope.

I was able to make this work with introducing a constructor and some help from @Sorin.



    #include <iostream>

    #include <vector>

    #include <string>





    struct A

    {

        std::string a;

        std::vector<A> avector;

        A(std::string aa)

        {

           a = aa;

        }

    };



    typedef std::vector<A> Avector;



    A& func(A& x)

    {

       return x.avector[0];

    }



    int main()

    {

       A genesis("Parent");

       genesis.avector.push_back(static_cast<A>("Child1"));

       genesis.avector.push_back(static_cast<A>("Child2"));



       A& l1 = genesis.avector[0];

       A& l2 = genesis.avector[1];



       std::cout << "l1: " << l1.a << std::endl;

       std::cout << "l2: " << l2.a << std::endl;



       A& lx = func(genesis);

       lx.a = "Childx";



       std::cout << "l1: " << genesis.avector[0].a << std::endl;

       std::cout << "l2: " << l2.a << std::endl;

       return 1;

    }

Output:

l1: Child1

l2: Child2

l1: Childx

l2: Child2