returning STL objects

Code:

vector<int> test() {
	vector<int> eg;
        [.......]
	return eg;
}    
vector<int> myvec = test();

Is this copied, or are references implicitly used?

In other words, if eg turns out to have 5000 elements and I call test in a loop a few thousand times, AND eg is created and then copied each time -- this is not so good.

I was hoping the magic of ref counting would mean that I could return a reference to eg, but I guess eg is still just a local variable, so it seems the only way around this is:

Code:

vector<int> *test() {
	vector<int> *eg = new vector<int>();
	return eg;
}

Or have I misunderstood something?

Originally Posted by MK27

Is this copied, or are references implicitly used?

The vector may or may not be copied, depending on whether named return value optimisation is performed by the compiler. That is, the compiler might be able to elide the copying if it can recognise that NRVO may be applied.

Neither references nor reference counting is involved here. Generally, if you do not want to rely on return value optimisation, you would just provide the function with a reference parameter.

Originally Posted by laserlight

Generally, if you do not want to rely on return value optimisation, you would just provide the function with a reference parameter.

Sounds good.

Well semantically, there is a copy of the whole vector going on.

But modern compilers should be doing this
Return value optimization - Wikipedia, the free encyclopedia

Edit:
0/2 vs. laserlight today

If you do it in a tight loop, it might be better performance-wise to move the creation of the vector out of the loop:

Code:

void test(vector<int>&);

int main()
{
    vector<int> vec;
    while (sth) {
        vec.clear(); //or test might clear it
        test(vec);
        ...
    }
}

Whatever you do, don't allocate the vector with new. It ain't that good performance-wise either, and it defeats the purpose of a vector as an automatically managed array (you'll need to make sure it is deleted).