How to accumulate template parameter pack? - c++

Question

Suppose I want to enable writing this:
template <int a,int b> struct add_base{ static const int value = a+b;};
template<int...a> using add = accumulate<add_base,0,a...>;
template <int a,int b> struct mult_base{ static const int value = a*b;};
template<int...a> using mult = accumulate<mult_base,1,a...>;
template <int a,int b> struct sqsum_base{ static const int value = a+b*b;};
template<int...a> using sqsum = accumulate<sqsum_base,0,a...>;
static_assert( add<1,2,3>::value == 6 );
static_assert( mult<2,2,2>::value == 8 );
static_assert( sqsum<1,2,3>::value == 14 );
My accumulate looks like this:
template <template <int,int> class G,
int first, int second,
int...more>
struct accumulate {
static const int value = accumulate<G,G<first,second>::value,more...>::value;
};
template <template <int,int> class G,
int first, int second>
struct accumulate<G,first,second> {
static const int value = G<first,second>::value;
};
Now I wonder if accumulate can be condensed by expanding the recusion inline, something like:
template <template <int,int> class G,
int first,int second,
int...more>
struct accumulate {
static const int value = G< G<first,second>::value , more...>::value;
};
This is wrong and will result in
error: Wrong number of template arguments (3 should be 2)
Is it possible to unpack the parameters to instantiate G recursively in one line? If not, how to write accumulate without having to write a specialization?

Answer 1

If your compiler has support for C++17 then you may want to utilize fold expression:
template<int ... x_item> struct
accumulate
{
static inline constexpr int const s_value{(0 + ... + x_item)};
};
static_assert(6 == accumulate<1, 2, 3>::s_value);
online compiler
Example of parametrized operation:
template<typename x_Op, int ... x_items> struct
accumulate
{
static inline constexpr int const s_value{(x_Op{0} + ... + x_Op{x_items}).value};
};
struct
sq_sum
{
int value;
};
inline constexpr sq_sum
operator +(sq_sum left, sq_sum right)
{
return sq_sum{left.value + right.value * right.value};
}
static_assert(14 == accumulate<sq_sum, 1, 2, 3>::s_value);
online compiler