对的重载>> istream迭代器[重复]

Question

我正在尝试在对上构造一个 ifstream_iterator。我的代码如下：

typedef pair<float, int> T;

istream& operator>>(istream& stream, T& in) {
    stream >> in.first >> in.second;
    return stream;
}


int main(int argc, char **argv) {
    ifstream infile("dummy2");
    istream_iterator<T> iit(infile);
    istream_iterator<T> eos;
    while (iit != eos) {
        cout << (*iit).first << endl;
        ++iit;
    }
    return 0;
}

我有一个虚拟文件“dummy2”如下：

"a" 4 "b" 5

我想输出对 {"a", 4} 和 {"b", 5}。

但是，我得到了令人讨厌的编译错误

In file included from /usr/include/c++/4.6/iterator:66:0,
                 from filestream.cpp:9:
/usr/include/c++/4.6/bits/stream_iterator.h: In member function ‘void std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::_M_read() [with _Tp = std::pair<float, int>, _CharT = char, _Traits = std::char_traits<char>, _Dist = long int]’:
/usr/include/c++/4.6/bits/stream_iterator.h:70:9:   instantiated from ‘std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_iterator(std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type&) [with _Tp = std::pair<float, int>, _CharT = char, _Traits = std::char_traits<char>, _Dist = long int, std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type = std::basic_istream<char>]’
filestream.cpp:32:33:   instantiated from here
/usr/include/c++/4.6/bits/stream_iterator.h:121:6: error: no match for ‘operator>>’ in ‘*((std::istream_iterator<std::pair<float, int> >*)this)->std::istream_iterator<std::pair<float, int> >::_M_stream >> ((std::istream_iterator<std::pair<float, int> >*)this)->std::istream_iterator<std::pair<float, int> >::_M_value’
/usr/include/c++/4.6/bits/stream_iterator.h:121:6: note: candidates are:
/usr/include/c++/4.6/istream:122:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:122:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}’
/usr/include/c++/4.6/istream:126:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__ios_type& (*)(std::basic_istream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>, std::basic_istream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/include/c++/4.6/istream:126:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/4.6/istream:133:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:133:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/4.6/istream:169:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:169:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘bool&’
/usr/include/c++/4.6/istream:173:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/istream:173:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘short int&’
/usr/include/c++/4.6/istream:176:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:176:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘short unsigned int&’
/usr/include/c++/4.6/istream:180:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/istream:180:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘int&’
/usr/include/c++/4.6/istream:183:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:183:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘unsigned int&’
/usr/include/c++/4.6/istream:187:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:187:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘long int&’
/usr/include/c++/4.6/istream:191:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:191:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘long unsigned int&’
/usr/include/c++/4.6/istream:196:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:196:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘long long int&’
/usr/include/c++/4.6/istream:200:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:200:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘long long unsigned int&’
/usr/include/c++/4.6/istream:205:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(float&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:205:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘float&’
/usr/include/c++/4.6/istream:209:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(double&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:209:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘double&’
/usr/include/c++/4.6/istream:213:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long double&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:213:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘long double&’
/usr/include/c++/4.6/istream:217:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]
/usr/include/c++/4.6/istream:217:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘void*&’
/usr/include/c++/4.6/istream:241:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
/usr/include/c++/4.6/istream:241:7: note:   no known conversion for argument 1 from ‘std::pair<float, int>’ to ‘std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
/usr/include/c++/4.6/bits/basic_string.tcc:998:5: note: template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/random.tcc:923:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::uniform_int_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:968:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::uniform_real_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:1890:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::cauchy_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.h:3381:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::bernoulli_distribution&)
/usr/include/c++/4.6/bits/random.tcc:1733:5: note: template<class _RealType1, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::normal_distribution<_RealType1>&)
/usr/include/c++/4.6/bits/random.tcc:1063:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::geometric_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:2085:5: note: template<class _RealType1, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::gamma_distribution<_RealType1>&)
/usr/include/c++/4.6/bits/random.tcc:1622:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::exponential_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:2143:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::weibull_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:2200:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::extreme_value_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:194:5: note: template<class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::linear_congruential_engine<_UIntType, __a, __c, __m>&)
/usr/include/c++/4.6/bits/random.tcc:491:5: note: template<class _UIntType, long unsigned int __w, long unsigned int __n, long unsigned int __m, long unsigned int __r, _UIntType __a, long unsigned int __u, _UIntType __d, long unsigned int __s, _UIntType __b, long unsigned int __t, _UIntType __c, long unsigned int __l, _UIntType __f, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d, __s, __b, __t, __c, __l, __f>&)
/usr/include/c++/4.6/bits/random.tcc:642:5: note: template<class _UIntType, long unsigned int __w, long unsigned int __s, long unsigned int __r, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::subtract_with_carry_engine<_UIntType, __w, __s, __r>&)
/usr/include/c++/4.6/bits/random.tcc:709:5: note: template<class _RandomNumberEngine, long unsigned int __p, long unsigned int __r, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::discard_block_engine<_RandomNumberEngine, __p, __r>&)
/usr/include/c++/4.6/bits/random.tcc:815:5: note: template<class _RandomNumberEngine, long unsigned int __k, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::shuffle_order_engine<_RandomNumberEngine, __k>&)
/usr/include/c++/4.6/bits/random.tcc:1138:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::negative_binomial_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:1328:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::poisson_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:1577:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::binomial_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:1782:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::lognormal_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:1827:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::chi_squared_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:1936:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::fisher_f_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:1981:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::student_t_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:2308:5: note: template<class _IntType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::discrete_distribution<_IntType>&)
/usr/include/c++/4.6/bits/random.tcc:2488:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::piecewise_constant_distribution<_RealType>&)
/usr/include/c++/4.6/bits/random.tcc:2690:5: note: template<class _RealType, class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::piecewise_linear_distribution<_RealType>&)
/usr/include/c++/4.6/bits/istream.tcc:957:5: note: template<class _CharT2, class _Traits2> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT2*)
/usr/include/c++/4.6/bits/istream.tcc:925:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT&)
/usr/include/c++/4.6/istream:709:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char&)
/usr/include/c++/4.6/istream:714:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char&)
/usr/include/c++/4.6/istream:756:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char*)
/usr/include/c++/4.6/istream:761:5: note: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char*)
/usr/include/c++/4.6/istream:852:5: note: std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&&, _Tp&) [with _CharT = char, _Traits = std::char_traits<char>, _Tp = std::pair<float, int>] <near match>
/usr/include/c++/4.6/istream:852:5: note:   no known conversion for argument 1 from ‘std::istream_iterator<std::pair<float, int> >::istream_type {aka std::basic_istream<char>}’ to ‘std::basic_istream<char>&&’
/usr/include/c++/4.6/iomanip:70:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Resetiosflags)
/usr/include/c++/4.6/iomanip:100:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Setiosflags)
/usr/include/c++/4.6/iomanip:131:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Setbase)
/usr/include/c++/4.6/iomanip:169:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Setfill<_CharT>)
/usr/include/c++/4.6/iomanip:199:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Setprecision)
/usr/include/c++/4.6/iomanip:229:5: note: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Setw)
/usr/include/c++/4.6/iomanip:263:5: note: template<class _CharT, class _Traits, class _MoneyT> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::_Get_money<_MoneyT>)
make: *** [filestream] Error 1

我无法完全理解这里发生的事情。任何帮助表示赞赏。 非常感谢！

Answer 1

您会注意到，只需这样做：

T t;
infile >> t;

像魅力一样工作！怎么了？

std::pair 在命名空间std 中声明。

istream_iterator<> 确实使用了operator>>，并且它在实例化时使用 ADL（这称为 两阶段查找）。

但是，在实例化时，它不会考虑您的 operator>>，因为它不存在于实现 istream_iterator 的命名空间中，也没有在任何隐含的关联命名空间中声明通过参数（这是 ADL 步骤）。

您可能会想通过在 std 命名空间中定义重载来“修复”它（并且它有效），但标准不允许这样做，并且技术上会导致 Undefined Behaviour。

所以你最好引入一种允许 ADL 发挥作用的类型：

struct T : public std::pair<int,float> {};

Live On Coliru

#include <utility>
#include <iostream>
#include <iterator>

struct T : std::pair<float, int> {
    using std::pair<float, int>::pair;
};

std::istream& operator>>(std::istream& stream, T& in) {
    return stream >> in.first >> in.second;
}

int main() {
    std::istream_iterator<T> iit(std::cin);
    std::istream_iterator<T> eos;
    while (iit != eos) {
        std::cout << (*iit).first << std::endl;
        ++iit;
    }
}

---

我怀疑从理论上讲，您应该能够通过重新排序包含并足够早地注入重载来欺骗事物，因此它在名称查找的第一阶段是“可见的”。但是，此选项是否可用取决于实现，如果您不走运，您仍然无法在不更改标准库头文件的情况下实现它（不要这样做强>)。

Answer 2

这是一个相当普遍的问题，没有真正干净的解决方案。通常可行的一种方法是将您的操作员放入namespace std：

typedef pair<float, int> T;

namespace std {
    istream& operator>>(istream& stream, T& in) {
        stream >> in.first >> in.second;
        return stream;
    }
}

int main(int argc, char **argv) {
    ifstream infile("dummy2");
    istream_iterator<T> iit(infile);
    istream_iterator<T> eos;
    while (iit != eos) {
        cout << (*iit).first << endl;
        ++iit;
    }
    return 0;
}

正式地，这给出了未定义的行为——你可以向namespace std 添加一些特化，但你不能在那里添加重载。非正式地，我还没有看到它导致问题的编译器/库。