c++ - 将 istream 运算符与类交朋友

Question

您好，我正在尝试将我的运算符重载>>到我的班级，但我在 Eclipse 中遇到错误。

代码：

    friend istream& operator>>(const istream& is, const RAngle& ra){
    return is >> ra.x >> ra.y;
}

代码2：

    friend istream& operator>>(const istream& is, const RAngle& ra)
{
    is >> ra.x;
    is >> ra.y;
    return is 
}

都崩溃了，我不知道为什么，请帮忙。

编辑：

ra.x&ra.y都是我班的 2 个私人整数；

完整错误：

错误：

   ..\/rightangle.h: In function 'std::istream& operator>>(std::istream&, const RAngle&)':
..\/rightangle.h:65:12: error: ambiguous overload for 'operator>>' in 'is >> ra.RAngle::x'
..\/rightangle.h:65:12: note: candidates are:
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note:   no known conversion for argument 1 from 'const int' to 'std::ios_base& (*)(std::ios_base&)'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}'
..\/rightangle.h:66:12: error: ambiguous overload for 'operator>>' in 'is >> ra.RAngle::y'
..\/rightangle.h:66:12: note: candidates are:
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note:   no known conversion for argument 1 from 'const int' to 'std::ios_base& (*)(std::ios_base&)'
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/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>] <near match>
c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note:   no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}''

Answer 1

一个问题在于操作员的签名：

friend istream& operator>>(const istream& is, const RAngle& ra){
//                         ^^^^^              ^^^^^
    return is >> ra.x >> ra.y;
}

如果参数是，const您将无法修改它们。从错误消息来看，它们似乎表明编译器无法为is >> ra.xand找到适当的重载is >> ra.y。这可能与上一个问题（找不到采用 const-lvalue 的重载）或不相关，具体取决于ra.xand的类型ra.y（问题中未显示）。

Answer 2

因为operator>>要修改你传递给它的参数，它们不能是对 const 的引用，所以你想要的是这样的：

friend std::istream &operator>>(std::istream &is, RAngle &ra) { 
    return is >> ra.x >> ra.y;
}

请注意，几乎所有 I/O 操作也（至少可能）更改流的状态，因此您（几乎？）总是希望通过非常量引用传递流参数。

Answer 3

请从参数“const RANgle& ra”中删除 const。您不能修改方法中的常量引用。