Quick compilation of large tuples

large_tuples.cpp

#include <iostream>
#include <tuple>
using namespace std;

// When tuples get large, compile times suffer because the compiler is hashing
// the name of a really long type. Lambdas offer a way to truncate that type
// name, leading to faster compile times and the ability to instantiate larger
// tuples (the template depth quickly exceeds the max).

// Compare compile times:

// $ time g++ -std=c++1y -DUSE_LAMBDAS_AS_TUPLES -g -O2 -c main.cpp -o main-lambda.o
//
// real	0m0.323s
// user	0m0.296s
// sys	0m0.020s

// $ time g++ -std=c++1y -ftemplate-depth=1024 -g -O2 -c main.cpp -o main-template.o
//
// real	0m0.833s
// user	0m0.780s
// sys	0m0.048s

#ifdef USE_LAMBDAS_AS_TUPLES

int main(void)
{
  auto list = [] (auto ...xs) {
    return [=] (auto access) { return access(xs...); };
  };

  auto length = [] (auto xs) {
    return xs([] (auto ...z) { return sizeof...(z); });
  };

  // A tuple of length 100!
  auto len = length(list( 1,  2,  3,  4,  5,  6,  7,  8,  9, 10,
                         11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
                         21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
                         31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
                         41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
                         51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
                         61, 62, 63, 64, 65, 66, 67, 68, 69, 70,
                         71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
                         81, 82, 83, 84, 85, 86, 87, 88, 89, 90,
                         91, 92, 93, 94, 95, 96, 97, 98, 99, 100));
  cout << len << endl;
}

#else

int main(void)
{
  // The compiler can't deal with a tuple much bigger than this (50)
  auto len = tuple_size<decltype(
      make_tuple( 1,  2,  3,  4,  5,  6,  7,  8,  9, 10,
                 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
                 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
                 31, 33, 33, 34, 35, 36, 37, 38, 39, 40,
                 41, 44, 44, 44, 44, 46, 47, 48, 49, 50))>::value;
  cout << len << endl;
}

#endif