How to accumulate template parameter pack?

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?

user7860670 · Accepted Answer · 2018-09-16T09:51:15.527

4

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

edited Sep 16 '18 at 09:51

answered Sep 16 '18 at 09:33

user7860670

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nice. I would like to parametrize the "`accu+=elem`" similar to `std::accumulate`, thats why i use this `add_base` and `mutl_base`, is this possible somehow? – 463035818_is_not_an_ai Sep 16 '18 at 09:37
eg with `sqrsum_base::value = (a + b*b)` i could calculate the squared sum – 463035818_is_not_an_ai Sep 16 '18 at 09:39
@user463035818 Since folding is performed on binary operator the way to achieve such a behavior would be to define a custom binary operator by wrapping operands into some proxy class. – user7860670 Sep 16 '18 at 09:45
isnt this what I do in my example? how would you calculate the squared sum of the pack? – 463035818_is_not_an_ai Sep 16 '18 at 09:47
i am stuck with c++14 for the moment :/ – 463035818_is_not_an_ai Sep 16 '18 at 09:49
@user463035818 I've updated my answer with parametrization example. – user7860670 Sep 16 '18 at 09:51
then I guess in c++14 what I have is already more or less fine, or is there some simple trick to emulate fold expressions? – 463035818_is_not_an_ai Sep 16 '18 at 09:58
@user463035818 I think with C++14 it should be possible to write constexpr functions where unfolding is performed by expanding template parameter pack to initialize a local array variable. – user7860670 Sep 16 '18 at 10:01

How to accumulate template parameter pack?

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