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#include <iostream>
#include <string>
#include <type_traits>
using namespace std;
 
 
string to_string(const string& s) { return s; }
string to_string(const char* s) { return s; }
 
 
template<typename... Args>
string stringer(Args&&... args)
{
    std::string result;
    using std::to_string;
    int unpack[]{0, (result += to_string(forward<Args>(args)), 0)...};
    static_cast<void>(unpack);
    return result;
}
 
int main() {
	string a = stringer("a");
	string b = stringer(1);
	string c = stringer(a, b);
	string d = stringer(1, a, 2, b, 3.0);
	cout << a << endl;
	cout << b << endl;
	cout << c << endl;
	cout << d << endl;
	return 0;
}

prog.cpp: In instantiation of ‘std::string stringer(Args&& ...) [with Args = const char (&)[2]; std::string = std::basic_string<char>]’:
prog.cpp:22:25:   required from here
prog.cpp:16:61: error: call of overloaded ‘to_string(const char [2])’ is ambiguous
     int unpack[]{0, (result += to_string(forward<Args>(args)), 0)...};
                                                             ^
prog.cpp:16:61: note: candidates are:
In file included from /usr/include/c++/4.8/string:52:0,
                 from /usr/include/c++/4.8/bits/locale_classes.h:40,
                 from /usr/include/c++/4.8/bits/ios_base.h:41,
                 from /usr/include/c++/4.8/ios:42,
                 from /usr/include/c++/4.8/ostream:38,
                 from /usr/include/c++/4.8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note: std::string std::to_string(int) <near match>
   to_string(int __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘int’
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note: std::string std::to_string(unsigned int) <near match>
   to_string(unsigned __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note: std::string std::to_string(long int) <near match>
   to_string(long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘long int’
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note: std::string std::to_string(long unsigned int) <near match>
   to_string(unsigned long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘long unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note: std::string std::to_string(long long int) <near match>
   to_string(long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘long long int’
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note: std::string std::to_string(long long unsigned int) <near match>
   to_string(unsigned long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note:   no known conversion for argument 1 from ‘const char [2]’ to ‘long long unsigned int’
prog.cpp: In instantiation of ‘std::string stringer(Args&& ...) [with Args = std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::basic_string<char, std::char_traits<char>, std::allocator<char> >&; std::string = std::basic_string<char>]’:
prog.cpp:24:26:   required from here
prog.cpp:16:61: error: no matching function for call to ‘to_string(std::basic_string<char>&)’
     int unpack[]{0, (result += to_string(forward<Args>(args)), 0)...};
                                                             ^
prog.cpp:16:61: note: candidates are:
In file included from /usr/include/c++/4.8/string:52:0,
                 from /usr/include/c++/4.8/bits/locale_classes.h:40,
                 from /usr/include/c++/4.8/bits/ios_base.h:41,
                 from /usr/include/c++/4.8/ios:42,
                 from /usr/include/c++/4.8/ostream:38,
                 from /usr/include/c++/4.8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note: std::string std::to_string(int)
   to_string(int __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘int’
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note: std::string std::to_string(unsigned int)
   to_string(unsigned __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note: std::string std::to_string(long int)
   to_string(long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long int’
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note: std::string std::to_string(long unsigned int)
   to_string(unsigned long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note: std::string std::to_string(long long int)
   to_string(long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long long int’
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note: std::string std::to_string(long long unsigned int)
   to_string(unsigned long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long long unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2898:3: note: std::string std::to_string(float)
   to_string(float __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2898:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘float’
/usr/include/c++/4.8/bits/basic_string.h:2907:3: note: std::string std::to_string(double)
   to_string(double __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2907:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘double’
/usr/include/c++/4.8/bits/basic_string.h:2916:3: note: std::string std::to_string(long double)
   to_string(long double __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2916:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long double’
prog.cpp: In instantiation of ‘std::string stringer(Args&& ...) [with Args = int, std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, int, std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, double; std::string = std::basic_string<char>]’:
prog.cpp:25:37:   required from here
prog.cpp:16:61: error: no matching function for call to ‘to_string(std::basic_string<char>&)’
     int unpack[]{0, (result += to_string(forward<Args>(args)), 0)...};
                                                             ^
prog.cpp:16:61: note: candidates are:
In file included from /usr/include/c++/4.8/string:52:0,
                 from /usr/include/c++/4.8/bits/locale_classes.h:40,
                 from /usr/include/c++/4.8/bits/ios_base.h:41,
                 from /usr/include/c++/4.8/ios:42,
                 from /usr/include/c++/4.8/ostream:38,
                 from /usr/include/c++/4.8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note: std::string std::to_string(int)
   to_string(int __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2864:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘int’
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note: std::string std::to_string(unsigned int)
   to_string(unsigned __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2869:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note: std::string std::to_string(long int)
   to_string(long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2875:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long int’
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note: std::string std::to_string(long unsigned int)
   to_string(unsigned long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2880:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note: std::string std::to_string(long long int)
   to_string(long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2886:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long long int’
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note: std::string std::to_string(long long unsigned int)
   to_string(unsigned long long __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2892:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long long unsigned int’
/usr/include/c++/4.8/bits/basic_string.h:2898:3: note: std::string std::to_string(float)
   to_string(float __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2898:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘float’
/usr/include/c++/4.8/bits/basic_string.h:2907:3: note: std::string std::to_string(double)
   to_string(double __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2907:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘double’
/usr/include/c++/4.8/bits/basic_string.h:2916:3: note: std::string std::to_string(long double)
   to_string(long double __val)
   ^
/usr/include/c++/4.8/bits/basic_string.h:2916:3: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘long double’

Standard output is empty