Question

我尝试编写代码来从名为“test.txt”的文件中读取字符串，并将字符串写入标准输出。以下代码效果很好：

 int main()
 {
     using namespace std;
     ifstream file("test.txt");
     copy(istream_iterator<string>(file), 
          istream_iterator<string>(), 
          ostream_iterator<string>(cout, " ")); 
 }

但是，通过此修改，代码不再编译：

   int main()
   {
        using namespace std;
        copy(istream_iterator<string>(ifstream("test.txt")),  // <-- Error here
        istream_iterator<string>(), 
        ostream_iterator<string>(cout, " "));   
   }

为什么这个版本不能编译？

我使用的编译器是g ++ 4.6.2，错误如下：

ex11-16.cpp:16:65: error: no matching function for call to 'std::istream_iterator<std::basic_string<char> >::istream_iterator(std::ifstream)'
ex11-16.cpp:16:65: note: candidates are:
.../bits/stream_iterator.h:72:7: note: std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_iterator(const std::istream_iterator<_Tp, _CharT, _Traits, _Dist>&) [with _Tp = std::basic_string<char>, _CharT = char, _Traits = std::char_traits<char>, _Dist = int, std::istream_iterator<_Tp, _CharT, _Traits, _Dist> = std::istream_iterator<std::basic_string<char> >]
.../bits/stream_iterator.h:72:7: note:   no known conversion for argument 1 from 'std::ifstream {aka std::basic_ifstream<char>}' to 'const std::istream_iterator<std::basic_string<char> >&'
.../bits/stream_iterator.h:68:7: note: std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_iterator(std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type&) [with _Tp = std::basic_string<char>, _CharT = char, _Traits = std::char_traits<char>, _Dist = int, std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_type = std::basic_istream<char>]
.../bits/stream_iterator.h:68:7: note:   no known conversion for argument 1 from 'std::ifstream {aka std::basic_ifstream<char>}' to 'std::istream_iterator<std::basic_string<char> >::istream_type& {aka std::basic_istream<char>&}'
.../bits/stream_iterator.h:64:26: note: std::istream_iterator<_Tp, _CharT, _Traits, _Dist>::istream_iterator() [with _Tp = std::basic_string<char>, _CharT = char, _Traits = std::char_traits<char>, _Dist = int]
.../bits/stream_iterator.h:64:26: note:   candidate expects 0 arguments, 1 provided

Answer 1

这里有（我相信）两个错误：

（1）你需要在test.txt附近加上引号：

istream_iterator<string>(ifstream("test.txt"), ... );

（2）istream_iterator的构造函数接受istream&（即对流的左值引用）。因此，您传入的流必须是左值。但是，传入ifstream("test.txt")传递的是ifstream类型的临时对象，它是 rvalue 而不是左值。这与你不能这样做的原因相同：

int function(int& x) {
    x++;
}
int main() {
    function(137); // Error - 137 is an rvalue, but lvalue is needed.
}

希望这有帮助！

为什么istream_iterator <string>（ifstream（“test.txt”））会导致错误？</string>

1 个答案: