Call lambda with the cartesian product of values in multiple input vectors

Question

I have several vectors of either ints or doubles:

std::vector<int>    iv = { 1, 2, 3, 4 };
std::vector<double> jv = { .5, 1., 1.5, 2. };
std::vector<int>    kv = { 5, 4, 3, 2 };

I need to process the cartesian product of each vector:

for (int i : iv)
{
    for (double j : jv)
    {
        for (int k : kv)
        {
            process(i, j, k);
        }
    }
}

I would like to flatten this into a single call

product(iv, jv, kv, [=](int i, double j, int k)
    {
        process(i, j, k);
    });

• The number of input vectors is variable
• The types stored in the input vectors are variable

Is this possible? (I'm using C++14)

Answer 1

Here's a short recursive version that just works with any iterables. It takes everything by const& for simplicity:

template <typename F>
void product(F f) {
    f();
}

template <typename F, typename C1, typename... Cs> 
void product(F f, C1 const& c1, Cs const&... cs) {
    for (auto const& e1 : c1) {
        product([&](auto const&... es){
            f(e1, es...);
        }, cs...);
    }   
}

Which would be:

product(process, iv, jv, kv);