CODEWITHSUNDEEP
c++c++11c++14variadic-templates
Patryk
Patryk

Reputation: 24140

Variadic templates give errors on more than one parameter

I am trying out the code from http://florianjw.de/en/variadic_templates.html:

#include <tuple>
#include <initializer_list>
#include <iostream>
#include <string>

template<typename Fun, typename...Ts>
void sequential_foreach(Fun f, const Ts&... args) {
  (void) std::initializer_list<int>{
    [&](const auto& arg){f(arg); return 0;}(args)...
  };
}

template<typename...Ts>
void print_all(std::ostream& stream, const Ts&... args) {
  sequential_foreach([&](const auto& arg){stream << arg;}, args...);
}

int main()
{
  std::string s1("string1");
  std::string s2("string2");
  print_all(std::cout, s1, s2);

  return 0;
}

But I am getting the following errors: (when I pass more than one parameter to print_all)

main.cpp: In instantiation of ‘void sequential_foreach(Fun, const Ts& ...) [with Fun = print_all(std::ostream&, const Ts& ...) [with Ts = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; std::ostream = std::basic_ostream<char>]::<lambda(const auto:2&)>; Ts = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >}]’:
main.cpp:14:67:   required from ‘void print_all(std::ostream&, const Ts& ...) [with Ts = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; std::ostream = std::basic_ostream<char>]’
main.cpp:21:30:   required from here
main.cpp:8:5: error: uninitialized const member ‘sequential_foreach(Fun, const Ts& ...) [with Fun = print_all(std::ostream&, const Ts& ...) [with Ts = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >}; std::ostream = std::basic_ostream<char>]::<lambda(const auto:2&)>; Ts = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >}]::<lambda(const auto:1&)>::<f capture>’
     [&](const auto& arg){f(arg); return 0;}(args)...
     ^

What am I doing wrong? ideone link: https://ideone.com/

I tried with ideone (gcc5.1 c++14) and on Redhat 6 gcc version 4.9.1 20140922 (Red Hat 4.9.1-10)

Upvotes: 3

Views: 146

Answers (1)

Patryk
Patryk

Reputation: 24140

As T.C. hasn't posted this as an answer I am posting it here so that everyone can find it.

ideone link

#include <tuple>
#include <initializer_list>
#include <iostream>
#include <string>
#include <vector>

template<typename Fun, typename...Ts>
void sequential_foreach(Fun f, const Ts&... args) {
  auto fun = [&](const auto& arg){f(arg); return 0;};
  (void) std::initializer_list<int>{ fun(args)... };
}

template<typename...Ts>
void print_all(std::ostream& stream, const Ts&... args) {
  sequential_foreach(
      [&](const auto& arg){stream << arg;},
      args...
      );
}

int main()
{
  std::string s1("string1");
  std::string s2("string2");
  print_all(std::cout, s1, s2);

  return 0;
}

Upvotes: 1

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