Easy way to iterate through multiple std::vectors in C++

Question

I have two std::vectors of same type, and I need to iterate through both of them using the same routine. Something like this:

std::vector<int> values1, values2;
int counter = 0;
for (int val : values1) counter += val;
for (int val : values2) counter += val;

Is there any simple way to write the last two lines in a single loop, to avoid code repetition? Something that would look like this:

std::vector<int> values1, values2;
int counter = 0;
for (int val : values1, values2) counter += val;

Answer 1

If you don't need to access either vector after iterating over them, you could move them into a std::initializer_list and then iterate over that:

std::vector<int> values1{1, 2, 3};
std::vector<int> values2{4, 5, 6};

for (const auto& v : {std::move(values1), std::move(values2)}) {
    for (auto value : v) {
        std::cout << value << ' ';
    }
}

// prints "1 2 3 4 5 6 "

---

As ShadowRanger pointed out below, moving from the original vectors isn't necessary if we instead iterate over an initializer list of std::reference_wrappers. This can be done by swapping std::move with std::cref (or std::ref if you need to mutate them):

for (const auto& v : {std::cref(values1), std::cref(values2)}) {
    for (auto value : v.get()) {
        std::cout << value << ' ';
    }
}
// prints "1 2 3 4 5 6 ", as before

Answer 2

No, there isn't, sorry. At least not directly in the language.

Boost can do it:

for (int val : boost::range::join(values1, values2))
   counter += val;

Because Boost can do it, you could make something to do it as well, by making an iterator type of your own that casts a "view" over both collections.

But, particularly in the simple case you've shown, it's often not worth it. If your loop body is more complex, I recommend hiving it off into a function that takes int. This can just be a lambda declared immediately above the loop. That's what I do.