问题
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 hardcoding this value I would like to write a constexpr function that will take the std::vector and return the depth as an unsigned integer value.
For example:
std::vector<std::vector<std::vector<int>>> v =
{
{ { 0, 1}, { 2, 3 } },
{ { 4, 5}, { 6, 7 } },
};
// Returns 3
size_t depth = GetDepth(v);
This needs to be done at compile time though because this depth will be passed to the template function as a template parameter:
// Same as calling foo<3>(v);
foo<GetDepth(v)>(v);
Is there any way to do this?
回答1:
A classic templating problem. Here's a simple solution like how the C++ standard library does. The basic idea is to have a recursive template that will count one by one each dimension, with a base case of 0 for any type that is not a vector.
#include <vector>
#include <type_traits>
template<typename T>
struct dimensions : std::integral_constant<std::size_t, 0> {};
template<typename T>
struct dimensions<std::vector<T>> : std::integral_constant<std::size_t, 1 + dimensions<T>::value> {};
template<typename T>
inline constexpr std::size_t dimensions_v = dimensions<T>::value; // (C++17)
So then you could use it like so:
dimensions<vector<vector<vector<int>>>>::value; // 3
// OR
dimensions_v<vector<vector<vector<int>>>>; // also 3 (C++17)
Edit:
Ok, I've finished the general implementation for any container type. Note that I defined a container type as anything that has a well-formed iterator type as per the expression begin(t) where std::begin is imported for ADL lookup and t is an lvalue of type T.
Here's my code along with comments to explain why stuff works and the test cases I used. Note, this requires C++17 to compile.
#include <iostream>
#include <vector>
#include <array>
#include <type_traits>
using std::begin; // import std::begin for handling C-style array with the same ADL idiom as the other types
// decide whether T is a container type - i define this as anything that has a well formed begin iterator type.
// we return true/false to determing if T is a container type.
// we use the type conversion ability of nullptr to std::nullptr_t or void* (prefers std::nullptr_t overload if it exists).
// use SFINAE to conditionally enable the std::nullptr_t overload.
// these types might not have a default constructor, so return a pointer to it.
// base case returns void* which we decay to void to represent not a container.
template<typename T>
void *_iter_elem(void*) { return nullptr; }
template<typename T>
typename std::iterator_traits<decltype(begin(*(T*)nullptr))>::value_type *_iter_elem(std::nullptr_t) { return nullptr; }
// this is just a convenience wrapper to make the above user friendly
template<typename T>
struct container_stuff
{
typedef std::remove_pointer_t<decltype(_iter_elem<T>(nullptr))> elem_t; // the element type if T is a container, otherwise void
static inline constexpr bool is_container = !std::is_same_v<elem_t, void>; // true iff T is a container
};
// and our old dimension counting logic (now uses std:nullptr_t SFINAE logic)
template<typename T>
constexpr std::size_t _dimensions(void*) { return 0; }
template<typename T, std::enable_if_t<container_stuff<T>::is_container, int> = 0>
constexpr std::size_t _dimensions(std::nullptr_t) { return 1 + _dimensions<typename container_stuff<T>::elem_t>(nullptr); }
// and our nice little alias
template<typename T>
inline constexpr std::size_t dimensions_v = _dimensions<T>(nullptr);
int main()
{
std::cout << container_stuff<int>::is_container << '\n'; // false
std::cout << container_stuff<int[6]>::is_container<< '\n'; // true
std::cout << container_stuff<std::vector<int>>::is_container << '\n'; // true
std::cout << container_stuff<std::array<int, 3>>::is_container << '\n'; // true
std::cout << dimensions_v<std::vector<std::array<std::vector<int>, 2>>>; // 3
}
回答2:
Assuming that a container is any type that has value_type and iterator member types (standard library containers satisfy this requirement) or a C-style array, we can easily generalize Cruz Jean's solution:
template<class T, typename = void>
struct rank : std::integral_constant<std::size_t, 0> {};
// C-style arrays
template<class T>
struct rank<T[], void>
: std::integral_constant<std::size_t, 1 + rank<T>::value> {};
template<class T, std::size_t n>
struct rank<T[n], void>
: std::integral_constant<std::size_t, 1 + rank<T>::value> {};
// Standard containers
template<class T>
struct rank<T, std::void_t<typename T::iterator, typename T::value_type>>
: std::integral_constant<std::size_t, 1 + rank<typename T::value_type>::value> {};
int main() {
using T1 = std::list<std::set<std::array<std::vector<int>, 4>>>;
using T2 = std::list<std::set<std::vector<int>[4]>>;
std::cout << rank<T1>(); // Output : 4
std::cout << rank<T2>(); // Output : 4
}
Container types can be further restricted if needed.
回答3:
You can define the following class template vector_depth<> which matches any type:
template<typename T>
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<T>:
template<typename T>
struct vector_depth<std::vector<T>> {
static constexpr size_t value = 1 + vector_depth<T>::value;
};
This specialization matches an std::vector<T> and corresponds to the recursive case.
Finally, define the function template, GetDepth(), that resorts to the class template above:
template<typename T>
constexpr auto GetDepth(const std::vector<T>& vec) {
return 1 + vector_depth<T>::value;
}
Example:
auto main() -> int {
std::vector<int> a;
std::vector<std::vector<int>> b;
std::vector<std::vector<std::vector<int>>> c;
// constexpr - depths are determinted at compile time
constexpr auto depth_a = GetDepth(a);
constexpr auto depth_b = GetDepth(b);
constexpr auto depth_c = GetDepth(c);
std::cout << depth_a << ' ' << depth_b << ' ' << depth_c << '\n';
}
The output of this program is:
1 2 3
来源：https://stackoverflow.com/questions/59490698/how-can-i-get-the-depth-of-a-multidimensional-stdvector-at-compile-time