c ++ getline从函数调用时编译错误 [英] c++ getline compile error when called from function

问题描述

//为什么这样工作？

// why does this work?

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main() {
  ifstream inputStream("scores.txt");
  string line;
  getline(inputStream, line);
  cout << line;
  return 0;
}

//但是会因编译器错误而失败。

// but this fails with compiler errors.

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

ifstream GetStream() {
  ifstream inputStream("scores.txt");
  return inputStream;
}

int main() {
  ifstream inputStream = GetStream();
  string line;
  getline(inputStream, line);
  cout << line;
  return 0;
}

在mac osx 10.6 w / g ++上编译4.2.1

compile on mac osx 10.6 w/ g++ 4.2.1

georges-iMac：cs106b george $ g ++ help.cpp -o help
/usr/include/c++/4.2.1/bits/ios_base.h：在复制构造函数'std :: basic_ios> :: basic_ios（const std :: basic_ios>&）'：
/usr/include/c++/4.2.1/bits/ios_base.h:779：error：'std :: ios_base： ：ios_base（const std :: ios_base&）'是private
/usr/include/c++/4.2.1/iosfwd:55：错误：在此上下文中
/usr/include/c++/4.2。 1 / iosfwd：在复制构造函数'std :: basic_ifstream> :: basic_ifstream（const std :: basic_ifstream>&）'：
/usr/include/c++/4.2.1/iosfwd:89：方法'std :: basic_ios> :: basic_ios（const std :: basic_ios>&）'首先需要这里
/usr/include/c++/4.2.1/streambuf：在复制构造函数'std :: basic_filebuf> :: basic_filebuf（const std :: basic_filebuf>&）'：
/usr/include/c++/4.2.1/streambuf:794：error：'std :: basic_streambuf< _CharT，_Traits> :: basic_streambuf const std :: basic_streambuf< _CharT，_Traits>&）[with _CharT = char，_Traits = std :: char_traits]'是私有的
/usr/include/c++/4.2.1/iosfwd:86：在这个上下文中
/usr/include/c++/4.2.1/iosfwd：在复制构造函数'std :: basic_ifstream> :: basic_ifstream（const std :: basic_ifstream>&）'：
/ usr /include/c++/4.2.1/iosfwd:89：note：合成方法'std :: basic_filebuf> :: basic_filebuf（const std :: basic_filebuf>&）'首先需要这里
help.cpp：In function 'std :: ifstream GetStream（）'：
help.cpp：9：note：合成方法'std :: basic_ifstream> :: basic_ifstream（const std :: basic_ifstream>&）' >

georges-iMac:cs106b george$ g++ help.cpp -o help /usr/include/c++/4.2.1/bits/ios_base.h: In copy constructor ‘std::basic_ios >::basic_ios(const std::basic_ios >&)’: /usr/include/c++/4.2.1/bits/ios_base.h:779: error: ‘std::ios_base::ios_base(const std::ios_base&)’ is private /usr/include/c++/4.2.1/iosfwd:55: error: within this context /usr/include/c++/4.2.1/iosfwd: In copy constructor ‘std::basic_ifstream >::basic_ifstream(const std::basic_ifstream >&)’: /usr/include/c++/4.2.1/iosfwd:89: note: synthesized method ‘std::basic_ios >::basic_ios(const std::basic_ios >&)’ first required here /usr/include/c++/4.2.1/streambuf: In copy constructor ‘std::basic_filebuf >::basic_filebuf(const std::basic_filebuf >&)’: /usr/include/c++/4.2.1/streambuf:794: error: ‘std::basic_streambuf<_CharT, _Traits>::basic_streambuf(const std::basic_streambuf<_CharT, _Traits>&) [with _CharT = char, _Traits = std::char_traits]’ is private /usr/include/c++/4.2.1/iosfwd:86: error: within this context /usr/include/c++/4.2.1/iosfwd: In copy constructor ‘std::basic_ifstream >::basic_ifstream(const std::basic_ifstream >&)’: /usr/include/c++/4.2.1/iosfwd:89: note: synthesized method ‘std::basic_filebuf >::basic_filebuf(const std::basic_filebuf >&)’ first required here help.cpp: In function ‘std::ifstream GetStream()’: help.cpp:9: note: synthesized method ‘std::basic_ifstream >::basic_ifstream(const std::basic_ifstream >&)’ first required here

推荐答案

问题出在这个函数：

ifstream GetStream() {
  ifstream inputStream("scores.txt");
  return inputStream;
}

您要传回 > by value 这是不允许的，因为它需要复制本地对象，但是你不能复制stream对象。通过将复制构造函数设为私有，可以禁止在C ++中复制任何流。任何方式任何，无论是 stringstream ， istream ， ostream ， iostream 或任何。阅读我的帖子这里为其残疾人的理由：

You're returning a stream object by value which is not allowed, because it requires copying of the local object, but you cannot make copy of stream object. Copying of any stream in C++ is disabled by having made the copy constructor private. Any means ANY, whether it is stringstream, istream, ostream, iostream or whatever. Read my post here for the rationale why its disabled:

• 为什么不允许复制stringstream？

• Why copying stringstream is not allowed?

可能希望通过引用返回它，但是您也不能通过引用返回它，因为 inputStream 是本地对象。

Now, you may want to return it by reference, but you cannot return it by reference also, because inputStream is a local object.

您可以尝试：

ifstream* GetStream() {
  return new ifstream("scores.txt");
}

int main() {
  ifstream *inputStream = GetStream();
  if ( inputStream && *inputStream )
  {
    string line;
    getline(*inputStream, line);
    cout << line;
  }
  if ( inputStream )
       delete inputStream; 
  return 0;
}

但我想知道为什么你会这样做。为什么不把一切都封装在一个类中？

But I'm wondering why you would do that. Why not encapsulate everything in a class?

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