問題與iostream_iterator與元組C++

Question

我想使用iostream_iterator元組。 我有超載輸入操作符>>元組。 但編譯器給編譯錯誤。

#include <vector> 
#include <iostream> 
#include <algorithm> 
#include <iterator> 
#include <tuple> 

using namespace std; 
using rec_t= tuple<int, string>; 

istream& operator>>(istream& strm, rec_t& r) { 
strm>>get<0>(r)>>get<1>(r); 
return strm; 
} 
int main() { 
    int n; 
    cin>>n; 
    vector<rec_t> ar(n); 
    copy_n(istream_iterator<rec_t>(cin), n, begin(ar)); 
    return 0; 
} 

編譯器錯誤信息是：

--------------體形：調試（編譯器：GNU GCC編譯器）-------- -------

g++ -Wall -fexceptions -std=c++11 -g -c main.cpp -o obj/Debug/main.o 
In file included from /usr/include/c++/4.9/iterator:66:0, 
       from main.cpp:4: 
/usr/include/c++/4.9/bits/stream_iterator.h: In instantiation of ‘void std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::_M_read() [with _Tp = std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >; _CharT = char; _Traits = std::char_traits<char>; _Dist = long int]’: 
/usr/include/c++/4.9/bits/stream_iterator.h:70:17: required from ‘std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_iterator(std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type&) [with _Tp = std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >; _CharT = char; _Traits = std::char_traits<char>; _Dist = long int; std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type = std::basic_istream<char>]’ 
main.cpp:18:39: required from here 
/usr/include/c++/4.9/bits/stream_iterator.h:121:17: error: cannot bind ‘std::istream_iterator<std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::istream_type {aka std::basic_istream<char>}’ lvalue to ‘std::basic_istream<char>&&’ 
     *_M_stream >> _M_value; 
       ^
In file included from /usr/include/c++/4.9/iostream:40:0, 
       from main.cpp:2: 
/usr/include/c++/4.9/istream:872:5: note: initializing argument 1 of ‘std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&&, _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >]’ 
    operator>>(basic_istream<_CharT, _Traits>&& __is, _Tp& __x) 
    ^
Process terminated with status 1 (0 minute(s), 1 second(s)) 
1 error(s), 3 warning(s) (0 minute(s), 1 second(s)) 

Answer 1

你已經成爲ADL「依賴於參數的查找」的犧牲品。

你應該去看看，因爲那裏有很好的解釋。 基本上，你的operator >>應該和你的rec_t在同一個命名空間。

但它不是 - rec_t在namespace std（因爲它確實是std::tuple<int, std::string>），並且operator >>位於全局名稱空間中。