Question

I come from Java, where filling containers did not involve thinking. My problem now that i work with c++ is, that filling a container in a function with data which is declared in function scope can lead to errors, where the data doesn’t exist any more when I want to access it.
I could not find tutorials addressing the problem, so I went the Java way and only made containers getting pointers declared with “new”. But now I am forced to return a

std::list<Vertex<float> >

from a function and thought this might be a good point to learn how I would fill and return such a thing. Would this

{
std::list<Vertex<float> > myList;
Vertex<float> v(0.0, 0.1, 0.2);
myList.push_back(v);
myList.push_back(Vertex<float>(1,0, 1.1, 1.2));
return myList;
}

actually be fine as a sample function body? And if yes, why would v still exist outside of the scope? Does each insertion in a container also imply a copying?

Answer 1

This would work “fine” since every operation there creates a copy.

• myList.push_back(v); creates a copy of v so that visiblity of v is now irrelevant.
• return myList; returns a copy of the list to the calling function so the visiblity of myList is now irrelevant. The calling function should make a copy of this list to keep it in scope, else it will be destroyed at the end of execution of the line that calls this function.

The reason that fine is quoted is that copies are typically expensive. In your case they are quite small so it might be irrelevant, and in many cases they might be optimised away, but it is still something to keep in mind.

Old C++ way of optimizing is to pass a list by reference and use that to construct your list, instead of returning by value.

void MakeMeAList(std::list<Vertex<float> >& aList){
    ....
}

std::list<Vertex<float> > aList;
MakeMeAList(aList);

As @billz suggests, Return Value Optimization should optimize away the copies even if this is not done.

New C++ (c++11) –
The use of emplace_back to construct the list, would be more efficient that copying, as long as the input variables are not going to be used any more. ( thanks @Troy)

My C++11 is weak, I am almost sure even returning by value is OK since Move semantics would optimize it away, but I am only 95% sure.