convert c multidimensional array to multidimensional c++ vector - c++

Question

i am trying to convert c multidimensional array to multidimensional c++ vector, i mean, to convert something like this int arr[2][3] = {{1,2,3}, {4,5,6}}; into the correspondent vector.
The array isn't necessarily 2D shaped, it could also be something like this:
int arr[2][2][3] = {
{
{1,2,3},
{4,5,6},
},
{
{7,8,9},
{10,11,12},
}
};
initially i thought that something like this would have worked, but it didn't really turned out to be the case because it seems like if std::vector doesn't allows conversions from C arrays.
std::vector<std::any> V(arr);
Then i thought at something like function recursion, this is my attempt, that (i don't know why!) throws error: no matching function for call to 'length' .
#include <iostream>
#include <type_traits>
#include <vector>
#include <any>
// Get the lenght of a classic C array.
template <class T, unsigned S>
inline unsigned length(const T (&v)[S]) {
return S;
};
// Check wether the input is a classic C array or not.
template <class T>
bool is(const T& t) {
return std::is_array_v<T>;
};
// Turn the classic C input array to vector.
template <class T>
std::vector<std::any> toVector(const T& t) {
std::vector<std::any> V;
for (int k = 0; k < length(t); k++) {
if (is(t[k])) {
V.push_back(toVector(t[k]));
} else {
V.push_back(t[k]);
}
}
return V;
}
int main() {
int16 a[] = {1,2,3};
auto b = toVector(a);
}
What did i wrong in the second attempt? Alternatively, is there a simpler way to manage to do this?
Also, i think it would be better to convert all the numbers in the vector to a unique given data type, is this possible?
I am using c++11 and g++ as compiler –
Note that i do not know how many dimensions my array has.

Answer 1

The equivalent of a C multidimensional array is a flat C++ vector plus a mumtidimensional array view.
The naive equivalent is a vector of vectors (maybe of vectors), which actually corresponds to a C "jagged" array. The memory layout and performance characteristics will be very different.
There are many multi dimensional array-view implementstions on the web.
template<class A>
struct raw_ptr { using type=A*; };
template<class A, std::size_t N>
struct raw_ptr<A[N]>:raw_ptr<A>{};
template<class A>
using raw_ptr_t = typename raw_ptr<A>::type;
template<class T>
struct array_view;
template<class T, std::size_t D0, std::size_t D1>
struct array_view<T[D0][D1]> {
T(*data)[D1]=0;
constexpr array_view( T(&arr)[D0][D1] ):data(arr) {}
explicit array_view( raw_ptr_t<T> buff ):data(reinterpret_cast<T(*)[D1]>(buff)) {}
constexpr array_view<T[D1]> operator[](std::size_t i)const{
return data[i];
}
};
template<class T, std::size_t D0>
struct array_view<T[D0]> {
constexpr array_view( T(&arr)[D0] ):data(arr) {}
explicit constexpr array_view( T* buff ):data(buff) {}
T* data=0;
constexpr T& operator[](std::size_t i)const{
return data[i];
}
};
to convert a int[4][5][6] you'd do:
int array[4][5][6]={/*whatever*/};
std::vector<int> buff(&array[0][0][0], &array[3][4][5]);
array_view<int[4][5][6]> view{buff.data()};
now, view[a][b][c] is the same as array[a][b][c].
Live example.
template<class Array>
struct wrap_array_in_vector {
using raw_ptr = raw_ptr_t<Array>;
using value_type = std::remove_pointer_t<raw_ptr>;
std::vector<value_type> data;
array_view<Array> view;
wrap_array_in_vector(wrap_array_in_vector const& other):
data(other.data),
view(data.data())
{}
wrap_array_in_vector(wrap_array_in_vector && other):
data(std::move(other.data)),
view(data.data())
{
other.view.data = nullptr; // no longer valid
}
decltype(auto) operator[](std::size_t i)const {
return view[i];
}
wrap_array_in_vector( Array const& arr ):
data( reinterpret_cast<value_type const*>(&arr[0]), reinterpret_cast<value_type const*>(&arr[1]) ),
view(data.data())
{}
};
template<class Array>
wrap_array_in_vector(Array&)->wrap_array_in_vector<Array>;
template<class Array>
wrap_array_in_vector(Array const&)->wrap_array_in_vector<Array>;
this lets you do
wrap_array_in_vector wrapped = array;
and wrapped deduces all of its type information it needs.

Answer 2

This should be close to optimal for conversion to a vector. The part of the description that was misleading was that the sizes are not regular, as they are list-initialized and in this case with int, will be zeros.
#include <type_traits>
#include <vector>
#include <iostream>
template<typename T, std::size_t N>
auto to_vectors( T const (&c_array)[N] ) {
using element_base_type = std::decay_t<T>;
if constexpr( std::is_array_v<T> ) {
using child_t = std::remove_cv_t<std::remove_reference_t<decltype( to_vectors( c_array[0] ) )>>;
auto result = std::vector<child_t>( );
result.reserve( N );
for( auto const & element: c_array ) {
result.push_back( to_vectors( element ) );
}
return result;
} else {
return std::vector<T>( c_array, c_array + N );
}
}
int arr[2][2][3] = {
{
{1,2,3},
{4,5,6},
},
{
{7,8,9},
}
};
auto v0 = to_vectors( arr );
template<typename Vec>
void display( Vec const & vec ) {
if constexpr( std::is_same_v<int, typename Vec::value_type> ) {
std::cout << "elements: {";
for( int e: vec ) {
std::cout << e << ',';
}
std::cout << "}\n";
} else {
std::cout << "element count: " << vec.size( ) << '\n';
for( auto const & child: vec ) {
display( child );
}
}
}
int main( ) {
display( v0 );
}
this will output
element count: 2
element count: 2
elements: {1,2,3,}
elements: {4,5,6,}
element count: 2
elements: {7,8,9,}
elements: {0,0,0,}