I am trying to make an N-dimensional array class that is dynamically resizable. My templated class has the signature ndarray<typename data_type, int dimensions>
. To hold the array I want to use a unique pointer class member to hold a templated vector of the types std::vector<type_t>
for 1D, std::vector<std::vector<type_t>>
for 2D, std::vector<std::vector<std::vector<type_t>>>
for 3d ect...
The unique pointer will have a different signature depending on the number of dimensions. I wanted to have the class constructor for ndarray to set the signature of the unique pointer and use new
to make the new vector but I would need an auto
member variable to be set to a vector of the type of the member variable which doesn't work. My current aproach is to use a templated function that returns an object of the type the array needs to be, template<typename data_type> auto dimension_helper(int dimensions)
and then set the unique pointer signature like this std::unique_ptr<decltype(<data_type>dimension_helper(dimens))> array
. This also does not work, giving me the errors template argument 1 is invalid and template argument 2 is invalid inside the unique pointer.
What can I do to make my existing code work, or is there a better way at approaching the problem in a similar way?
Code samples
header
#ifndef ND_H_
#define ND_H_
#include <cstring>
#include <vector>
#include <memory>
namespace NdArray{
//use the following for dimension types
template<typename T>
using d1 = std::vector<T>;
template<typename T>
using d2 = std::vector<d1<T>>;
template<typename T>
using d3 = std::vector<d2<T>>;
template<typename T>
using d4 = std::vector<d3<T>>;
template<typename T>
using d5 = std::vector<d4<T>>;
template<typename data_type>
auto dimension_helper(int dim);
template<typename data_type,int dimensions>
class ndarray{
int dims;
std::unique_ptr<std::vector<int>> xsub_spans;
public:
ndarray();
~ndarray();
std::unique_ptr<decltype(<data_type>dimension_helper(dimensions))> array;
template<typename dat_type, int dim>
friend std::ostream& operator<<(std::ostream& , ndarray<dat_type, dim>&);
};
}
#endif
definitions so far
#include "nd.h"
using namespace NdArray;
template<typename data_type, int dimensions>
ndarray<data_type, dimensions>::ndarray(){
dims = dimensions;
array = new <data_type>dimension_helper(dimensions);
}
template<typename data_type>
auto dimension_helper(){
switch (dims) {
case 1 : {
d1<data_type> type;
return type;
break;
}
case 2 : {
d2<data_type> type;
return type;
break;
}
case 3 : {
d3<data_type> type;
return type;
break;
}
case 4 : {
d4<data_type> type;
return type;
break;
}
case 5 : {
d5<data_type> type;
return type;
break;
}
}
}
A template argument list comes after the template's name, so:
dimension_helper<data_type>(dimensions)
instead of
<data_type>dimension_helper(dimensions)
However, your "dimension helper" has no chance to work, as the switch statement is a runtime branch, and fails to compile because the types of expressions in return statements are unequal. Instead, you can rewrite it as follows:
template <typename T>
struct identity { using type = T; };
template <typename T, std::size_t D>
struct dimension_helper : identity<std::vector<typename dimension_helper<T, D-1>::type>> {};
template <typename T>
struct dimension_helper<T, 0> : identity<T> {};
template <typename T, std::size_t D>
using dimension_helper_t = typename dimension_helper<T, D>::type;
and declare your pointer as:
std::unique_ptr<dimension_helper_t<data_type, dimensions>> array;
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