How can I get the depth of a multidimensional std::vector at compile time?

Question

I have a function that takes a multidimensional std::vector and requires the depth (or the number of dimensions) to be passed in as a template parameter. Instead of

Answer 1

You can define the following class template vector_depth<> which matches any type:

template
struct vector_depth {
   static constexpr size_t value = 0;
};

This primary template corresponds to the base case that ends the recursion. Then, define its corresponding specialization for std::vector:

template
struct vector_depth> {
   static constexpr size_t value = 1 + vector_depth::value;
};

This specialization matches an std::vector and corresponds to the recursive case.

Finally, define the function template, GetDepth(), that resorts to the class template above:

template
constexpr auto GetDepth(T&&) {
   return vector_depth>>::value;
}

Example:

auto main() -> int {
   int a{}; // zero depth
   std::vector b;
   std::vector> c;
   std::vector>> d;

   // constexpr - dimension determinted at compile time
   constexpr auto depth_a = GetDepth(a);
   constexpr auto depth_b = GetDepth(b);
   constexpr auto depth_c = GetDepth(c);
   constexpr auto depth_d = GetDepth(d);

   std::cout << depth_a << ' ' << depth_b << ' ' << depth_c << ' ' << depth_d;
}

The output of this program is:

0 1 2 3