Passing variadic parameters in an already template-variadic function - c++

Question

The title is bad but I couldn't come up with anything better. Feel free to change it.
Here's a template multidimensional array class that I'm currently working on. I'm trying to optimise it as much as I can:
#include <array>
template <typename T, std::size_t... Dimensions>
class multidimensional_array
{
public:
using value_type = T;
using size_type = std::size_t;
private:
template<typename = void>
static constexpr size_type multiply(void)
{
return 1u;
}
template<std::size_t First, std::size_t... Other>
static constexpr size_type multiply(void)
{
return First * multidimensional_array::multiply<Other...>();
}
public:
using container_type = std::array<value_type, multidimensional_array::multiply<Dimensions...>()>;
using reference = value_type &;
using const_reference = value_type const&;
using iterator = typename container_type::iterator;
private:
container_type m_data_array;
template<typename = void>
static constexpr size_type linearise(void)
{
return 0u;
}
template<std::size_t First, std::size_t... Other>
static constexpr size_type linearise(std::size_t index, std::size_t indexes...)
{
return multidimensional_array::multiply<Other...>()*index + multidimensional_array::linearise<Other...>(indexes);
}
public:
// Constructor
explicit multidimensional_array(const_reference value = value_type {})
{
multidimensional_array::fill(value);
}
// Accessors
reference operator()(std::size_t indexes...)
{
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
}
const_reference operator()(std::size_t indexes...) const
{
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
}
// Iterators
iterator begin()
{
return m_data_array.begin();
}
iterator end()
{
return m_data_array.end();
}
// Other
void fill(const_reference value)
{
m_data_array.fill(value);
}
};
My main function is
int main(void)
{
multidimensional_array<int, 2u, 3u, 4u, 5u, 6u> foo;
int k = 0;
for (auto& s : foo)
s = k++;
//std::cout << foo(0u, 0u, 0u, 1u, 0u) << std::endl;
return 0;
}
The above code compilers without warning/error. As soon as I uncomment the std::cout part though, I get this:
g++-7 -std=c++17 -o foo.o -c foo.cpp -Wall -Wextra -pedantic
foo.cpp: In instantiation of ‘multidimensional_array<T, Dimensions>::value_type& multidimensional_array<T, Dimensions>::operator()(std::size_t, ...) [with T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}; multidimensional_array<T, Dimensions>::reference = int&; multidimensional_array<T, Dimensions>::value_type = int; std::size_t = long unsigned int]’:
foo.cpp:99:37: required from here
foo.cpp:60:72: error: no matching function for call to ‘multidimensional_array<int, 2, 3, 4, 5, 6>::linearise<2, 3, 4, 5, 6>(std::size_t&)’
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
foo.cpp:38:30: note: candidate: template<class> static constexpr multidimensional_array<T, Dimensions>::size_type multidimensional_array<T, Dimensions>::linearise() [with <template-parameter-2-1> = <template-parameter-1-1>; T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}]
static constexpr size_type linearise(void)
^~~~~~~~~
foo.cpp:38:30: note: template argument deduction/substitution failed:
foo.cpp:60:72: error: wrong number of template arguments (5, should be at least 0)
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
foo.cpp:44:30: note: candidate: template<long unsigned int First, long unsigned int ...Other> static constexpr multidimensional_array<T, Dimensions>::size_type multidimensional_array<T, Dimensions>::linearise(std::size_t, std::size_t, ...) [with long unsigned int First = First; long unsigned int ...Other = {Other ...}; T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}]
static constexpr size_type linearise(std::size_t index, std::size_t indexes...)
^~~~~~~~~
foo.cpp:44:30: note: template argument deduction/substitution failed:
foo.cpp:60:72: note: candidate expects 2 arguments, 1 provided
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
Makefile:17: recipe for target 'foo.o' failed
make: *** [foo.o] Error 1
And I know why now. My question is, how can I fix linearise so that it can pass indexes without going through va_list and such? Unfortunately linearise is already a template, variadic function, so I can't use variadic template shenanigans in that regard.

Answer 1

As in preceding question the problem is that the following signatures
template<std::size_t First, std::size_t... Other>
static constexpr size_type linearise(std::size_t index,
std::size_t indexes...)
reference operator()(std::size_t indexes...)
const_reference operator()(std::size_t indexes...) const
aren't what do you mean (indexes a variadic list of std::size_t) but are exactly equivalent to
template<std::size_t First, std::size_t... Other>
static constexpr size_type linearise(std::size_t index,
std::size_t indexes,
...)
reference operator()(std::size_t indexes, ...)
const_reference operator()(std::size_t indexes, ...) const
where indexes is a single std::size_t followed by a C-style optional sequence of argument.
A simple solution (you tagged C++17 but is available starting from C++11) is based on the use of variadic templates.
By example, as follows
template <std::size_t First, std::size_t ... Other, typename ... Ts>
static constexpr size_type linearise (std::size_t index,
Ts ... indexes)
{ return multidimensional_array::multiply<Other...>() * index
+ multidimensional_array::linearise<Other...>(indexes...); }
// Accessors
template <typename ... Ts>
reference operator() (Ts ... indexes)
{ return m_data_array[
multidimensional_array::linearise<Dimensions...>(indexes...)]; }
template <typename ... Ts>
const_reference operator() (Ts ... indexes) const
{ return m_data_array[
multidimensional_array::linearise<Dimensions...>(indexes...)]; }
The following is you're code, modified and compilable
#include <array>
#include <iostream>
template <typename T, std::size_t ... Dimensions>
class multidimensional_array
{
public:
using value_type = T;
using size_type = std::size_t;
private:
template <typename = void>
static constexpr size_type multiply ()
{ return 1u; }
template <std::size_t First, std::size_t ... Other>
static constexpr size_type multiply(void)
{ return First * multidimensional_array::multiply<Other...>(); }
public:
using container_type = std::array<value_type,
multidimensional_array::multiply<Dimensions...>()>;
using reference = value_type &;
using const_reference = value_type const &;
using iterator = typename container_type::iterator;
private:
container_type m_data_array;
template <typename = void>
static constexpr size_type linearise ()
{ return 0u; }
template <std::size_t First, std::size_t ... Other, typename ... Ts>
static constexpr size_type linearise (std::size_t index,
Ts ... indexes)
{ return multidimensional_array::multiply<Other...>() * index
+ multidimensional_array::linearise<Other...>(indexes...); }
public:
// Constructor
explicit multidimensional_array (const_reference value = value_type{})
{ multidimensional_array::fill(value); }
// Accessors
template <typename ... Ts>
reference operator() (Ts ... indexes)
{ return m_data_array[
multidimensional_array::linearise<Dimensions...>(indexes...)]; }
template <typename ... Ts>
const_reference operator() (Ts ... indexes) const
{ return m_data_array[
multidimensional_array::linearise<Dimensions...>(indexes...)]; }
// Iterators
iterator begin ()
{ return m_data_array.begin(); }
iterator end ()
{ return m_data_array.end(); }
// Other
void fill (const_reference value)
{ m_data_array.fill(value); }
};
int main ()
{
multidimensional_array<int, 2u, 3u, 4u, 5u, 6u> foo;
int k{ 0 };
for ( auto & s : foo )
s = k++;
std::cout << foo(0u, 0u, 0u, 1u, 0u) << std::endl;
}
Bonus suggestion.
You tagged C++17 so you can use "folding".
So you can substitute the couple of multiply() template functions
template <typename = void>
static constexpr size_type multiply ()
{ return 1u; }
template <std::size_t First, std::size_t ... Other>
static constexpr size_type multiply ()
{ return First * multidimensional_array::multiply<Other...>(); }
with a single folded one
template <std::size_t ... Sizes>
static constexpr size_type multiply ()
{ return ( 1U * ... * Sizes ); }

Answer 2

My approach is similar to that in this answer, except that instead of using std::tuple to store a list of types, I define my own type size_t_pack to store a (compile-time) list of size_t's.
using std::size_t;
template<size_t... values>
struct size_t_pack{};
template<size_t first_value,size_t... rest_values>
struct size_t_pack<first_value,rest_values...>{
static constexpr size_t first=first_value;
using rest=size_t_pack<rest_values...>;
static constexpr size_t product=first*rest::product;
};
template<>struct size_t_pack<>{
static constexpr size_t product=1;
};
Defines members: first, rest (in case not empty) and product (since it's not possible to specialize a function using the templates of a template argument, as far as I know, another choice is to if constexpr and make the type support checking for empty)
With that, it's easy to define the linearize function:
template<class dimensions,class... SizeTs>
static constexpr size_type linearise(std::size_t index, SizeTs... indices)
{
using restDimensions=typename dimensions::rest;
return restDimensions::product *index +
multidimensional_array::linearise<restDimensions>(indices...);
}
Using a std::tuple to store the list of types (SizeTs) is also possible, although struct partial specialization is still required, as far as I know.

Answer 3

You need to make indexes a parameter pack by making the operator() function a template, and expand the parameter pack when you use it by putting ... afterwards:
template <class... DimensionType>
const_reference operator()(DimensionType... indexes) const
{
return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes...)];
}
See: parameter pack expansion
The code still will not compile because of a similar problem in linearize(), but that gets you on the right track.