c++ - Cross-platform "auto" keyword usage in lambdas: integral_constant as function argument -


i develop cross-platform (linux , win) library. within use of following helper templated functions work tuples (as example):

#include <iostream> #include <string> #include <tuple> #include <utility> #include <type_traits>  using namespace std;  template <class f, size_t... is> constexpr void index_apply_impl1(const f &f,                                 const index_sequence<is...>&) {     int d[] = { 0, (f(integral_constant<size_t, is> {}), 0)... }; }  template <size_t n, class f> constexpr void index_apply1(const f& f) {     index_apply_impl1(f, make_index_sequence<n>{}); }  template <class tuple, class f> constexpr void apply1(const tuple &t, const f &f) {     index_apply1<tuple_size<tuple>{}>(         [&](auto &is) { f(get<is>(t)); } ); }  template <size_t n, class f> constexpr void apply_by_index1(const f &f) {     index_apply1<n>(         [&](auto &&is) { f(integral_constant<size_t, is> {}); }); }  int main() {     auto t = make_tuple("aaa", 1, 11.11);      // not work gcc     //apply1(t, [&](auto &v) { cout << v << endl;} );      apply_by_index1<tuple_size<decltype(t)>::value>([&](auto &&i) { cout << get<i>(t) << endl; }); }

this code compiled , works expected gcc 5.4 , 6.4, won't compiled ms build tools 2015 (ms vc++ 2017 version not checked yet). error vc++ compiler prints related "auto"-type argument "is" of lambdas in function "apply_by_index1". vc++ says:

zz.cpp(29): error c2975: '_val': invalid template argument 'std::integral_constant', expected compile-time constant expression

...

zz.cpp(38): error c2672: 'get': no matching overloaded function found zz.cpp(29): note: see reference function template instantiation 'auto main::::operator ()>(std::integral_constant &&) const' being compiled

zz.cpp(38): error c2975: '_idx': invalid template argument 'std::get', expected compile-time constant expression

... , on

is generaly possible implement such functions vc++ or did miss something? thoughts replacing lambdas functors still have not idea how implement (i'm not proffesional programmer)

i have no clang/llvm belive code should compiled clang of version >=4. right?

i can give @ least partial solution. still cannot use std::get inside lambda.

namespace detail {     template <typename t, typename f, typename... ts, size_t... is>     void apply1(t&& t, f&& f, std::index_sequence<is...>)     {         const int d[] = { 0, (f(std::get<is>(t)), 0)... };     }     template <typename t, typename f, size_t... is>     void apply_by_index1(t&& t, f&& f, std::index_sequence<is...>)     {         const int d[] = { 0, (f(is), 0)... };     }      template <size_t i, typename... ts>     constexpr auto create_element(ts&&... ts)     {         return std::make_tuple(std::get<i>(std::forward<ts>(ts))...);     }      template <typename... ts, size_t... is>     constexpr auto merge(std::index_sequence<is...>, ts&&... ts)     {         return std::make_tuple(create_element<is>(std::forward<ts>(ts)...)...);     } }  template <typename t, typename f> void apply1(t&& t, f&& f) {     detail::apply1(std::forward<t>(t), std::forward<f>(f), std::make_index_sequence<std::tuple_size<std::decay_t<t>>::value>()); }  template <typename t, typename f> void apply_by_index1(t&& t, f&& f) {     detail::apply_by_index1(std::forward<t>(t), std::forward<f>(f), std::make_index_sequence<std::tuple_size<std::decay_t<t>>::value>()); }  template <typename t, typename... ts> constexpr auto merge(t&& t, ts&&... ts) {     return detail::merge(std::make_index_sequence<std::tuple_size<std::decay_t<t>>::value>(), std::forward<t>(t), std::forward<ts>(ts)...); }  int main() {     const auto t1 = std::make_tuple("hallo ", "hallo ", "hallo ");     const auto t2 = std::make_tuple("world", "jon", "peter");     const auto t3 = std::make_tuple("!", "!", "!");     const auto t4 = merge(t1, t2, t3);      apply1(t4, [](const auto& e)     {         std::cout << std::get<0>(e) << std::get<1>(e) << std::get<2>(e) << std::endl;     });      return 0; }

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