Return a large std::vector from a function

Question

In C++ 11, if I have a function that returns a std::vector<int> with a very large number of elements, does this return a deep or shallow copy of the vector which was originally created?

E.g.

std::vector<int> GetVec()
{
    std::vector<int> bar(1000000);
    // Fill in bar with numbers
    return bar;
};

int main()
{
    std::vector<int> foo = GetVec();
    return 0;
}

Will foo be a deep copy of the entire contents of bar, or will it just be a reference to bar?

My understanding is the C++ functions return copies, so in this case, how should I avoid having the make a costly copy of bar inside this function? I know that the elements of a std::vector are created on the heap, so I'm wondering if copying bar in the function creates a deep or shallow copy.

One way would be to pass foo as an argument:

void GetVec(std::vector<int>& bar)
{
    bar.resize(1000000);
    // Fill in bar with numbers
};

Another way would be to return by reference:

std::vector<int>& GetVec()
{
    std::vector<int> bar(1000000);
    // Fill in bar with numbers
    return bar;
};

But what's the best practice?

Your last snippet is not correct. You cannot return a reference to a temporary object. — Austin Brunkhorst, May 27 '15 at 17:14
Just copy by value. The returning will be optimized. Look up: Return value optimization (RVO), copy elision and move semantics. — , May 27 '15 at 17:16
For some additional information, see this duplicate as well: http://stackoverflow.com/questions/753312/returning-large-objects-in-functions — Bill Lynch, May 27 '15 at 17:17
@merlin2011 the elements are created on the heap even if you didn't use 'new' explicitly to instantiate the vector container — Marco A., May 27 '15 at 17:18
@MarcoA., I thought he was referring to the container object itself. I think I misread the question. — merlin2011, May 27 '15 at 17:21

Return a large std::vector from a function

0 Answers0