Add all parameters with parameter pack expansion

Question

Consider I have a variadic template with int... parameters. For example a function like this:

template<int... t>
int add(){
    return t... + ???
}

All the method should do is adding all the parameters. It can be easily achieved using recursive variadic templates. However, is it also possible expressing this (or something similar like using other binary operators to aggregate all the template parameters) using parameter pack expansion?

Okay, I just thought it might be even easier and better comprehensible with parameter pack expansion :) — gexicide, May 13 '14 at 10:39
The problem with the other solutions is they calculate the addition in execution time. — ABu, May 13 '14 at 21:46

rubenvb · Accepted Answer · 2016-06-16T12:29:51.990

18

Yes, using a trick I learnt from @Xeo in the Lounge. I originally used it to make a variadic "print" template function.

#include <iostream>

template<int... ints>
int add()
{
  int result = 0;
  using expand_variadic_pack  = int[]; // dirty trick, see below
  (void)expand_variadic_pack{0, ((result += ints), void(), 0)... };
  // first void: silence variable unused warning
  // uses braced-init-list initialization rules, which evaluates
  //  the elements inside a braced-init-list IN ORDER, to repetetively
  //  execute a certain operation
  // second void is to prevent malicious "operator," overloads, which
  //  cannot exist for void types
  // 0 at the end is to handle empty variadic pack (zero-size array initializer is illegal.
  return result;
}

int main()
{
  std::cout << add<1,2,3,4>() << '\n';
}

This works on every compiler that has decent C++11 support (GCC 4.8+, Clang 3.2+, MSVS2013, ...)

edited Jun 16 '16 at 12:29

answered May 13 '14 at 11:54

rubenvb

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Not sure whether to laugh or cry – Lightness Races in Orbit May 13 '14 at 12:09
@Lightness surely you've abused side-effects of expressions before in c++. – rubenvb May 13 '14 at 12:15
Of course! – Lightness Races in Orbit May 13 '14 at 12:17
2

This should also work (and look a bit cleaner) with `auto a = {(ret += ints)... }; (void)a;` – PlasmaHH May 14 '14 at 09:23
What's the first 0 within brace-init-list used for? – ABu May 13 '15 at 17:48
1

@Peregring-lk if the parameter pack is empty, a zero-sized array is initialized, which is illegal. For a more thorough explanation of what is really going on, see [this answer](http://stackoverflow.com/questions/17339789/how-to-call-a-function-on-all-variadic-template-args?lq=1). – rubenvb May 14 '15 at 17:44
This approach also works for variadic input parameters, eg: `template int add(T... ints)`: https://ideone.com/UtS43M – Remy Lebeau Mar 26 '20 at 22:18

score 8 · Answer 2 · answered May 13 '14 at 18:28

8

One possible variant which uses lambda and std::accumulate:

#include <array>
#include <numeric>

template <int... t>
int add()
{
    return [](const std::array<int, sizeof...(t)>& a)
    {
        return std::accumulate(a.begin(), a.end(), 0);
    }({t...});
}

answered May 13 '14 at 18:28

Constructor

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This one looks clean and much less of a hack! Unfortunately, computations are unlikely to be optimized in compile time. – Tigran Saluev Jun 16 '16 at 08:06

Add all parameters with parameter pack expansion

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