BOOST_FOREACH and a vector

Question

I have a vector of Scenes, vector<Scene>. What is the correct way to iterate over the elements, as reference or not?

For example this:

BOOST_FOREACH(Scene scene, mScenes)
{
       .....
}

Does the macro copy the Scene for each iteration over the vector, or does it use a reference behind the scenes?

So is it any different from this:

BOOST_FOREACH(Scene& scene, mScenes)
{
       .....
}

Answer 1

BOOST_FOREACH behaves exactly as you tell him, by value, reference or const reference

Answer 2

Your first example does copy each Scene. Most likely, you want the second. With the first, you can modify scene and the vector will be unaffected. If you're not modifying the vector, you should use const Scene& scene. If you are, use Scene &.

Answer 3

Note that with C++11 you don't need to use BOOST_FOREACH (excepted with some boost libs that return a pair of iterators).

for(const Scene& scene: mScenes){}

The same rules apply as for BOOST_FOREACH : you most likely want to use const &, rarely &, and never copy.