Question

即使＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;＆lt;对于下面的映射重载了运算符，Feature.h由名为Session.C的类使用，我试图使用＆lt;＆lt;来打印映射tKeyboard的值。我是这一切的新手，这是一个非常古老的代码，我对这个问题感到困惑，你能不能提出一些建议。感谢它。

FEATURE.H

class Feature {
..
public:

 typedef map<string,KeyStroke,std::less<string>,std::allocator<std::pair<const string, KeyStroke> > > tKeyboard;
                               // It is a map of keys
                               // key is the name of the keystroke
      typedef  tKeyboard::const_iterator cKeysIter;
      typedef  tKeyboard::iterator KeysIter;
..
  private:
  friend ostream &operator<<(ostream &os,const tKeyboard &);
..
};

Feature.C

ostream &operator<<(ostream &os,const Feature::tKeyboard &k)
{
    os <<" Keyboard >\n";
    for (Feature::cKeysIter i=k.begin() ; i!= k.end() ; i++)
    {
        os << "key name:"<<(*i).first << " ="<< (*i).second ;
    }
    return os;
}

Session.C

 if (keyStrokeVal.empty()){
       Traced(3)<<"sendKey:Error, key "<<keyname<<" not defined or empty\n";
       Traced(7)<<"Printing Feature "<<mCurrentFeature->get_mName() <<" 's keyboard \n" << mCurrentFeature->get_mKeyboard();
       Traced(7)<<"\n";
       return(-1);
    }

错误：

Session.C:541: error: no match for âoperator<<â in âstd::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)((std::ostream*)LogMgr::traceRovr())->std::basic_ostream<_CharT, _Traits>::operator<< [with _CharT = char, _Traits = std::char_traits<char>](std::endl [with _CharT = char, _Traits = std::char_traits<char>]))), ((const char*)"Session.C")))), ((const char*)" ")))->std::basic_ostream<_CharT, _Traits>::operator<< [with _CharT = char, _Traits = std::char_traits<char>](541))), ((const char*)" - ")))->std::basic_ostream<_CharT, _Traits>::operator<< [with _CharT = char, _Traits = std::char_traits<char>](out_time))), ((const char*)"Printing Feature ")))), ((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)((const std::string*)((Session*)this)->Session::mCurrentFeature->Feature::get_mName()))))), ((const char*)" \'s keyboard \012")) << ((Session*)this)->Session::mCurrentFeature->Feature::get_mKeyboard()â
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:108: note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>& (*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:117: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>& (*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:127: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:165: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:169: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:173: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/ostream.tcc:91: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:180: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/ostream.tcc:105: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:191: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:200: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:204: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:209: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:213: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:221: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/ostream:225: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/ostream.tcc:119: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>]

Answer 1

问题是如果friend声明首先声明（但没有定义）一个函数，那么该函数的名称不会引入周围的作用域。引用C ++ 11，[namespace.memdef]§3：

如果非本地类中的friend声明首先声明一个类或函数，那么友元类或函数是最内层封闭命名空间的成员。 无法通过非限定查找（3.4.1）或限定查找（3.4.3）找到朋友的名称，直到在该命名空间范围内提供匹配声明（在类定义之前或之后）给予友谊）。如果调用了友元函数，则可以通过名称查找找到其名称，该名称查找考虑名称空间中的函数和与函数参数类型相关联的类（3.4.2）。

（强调我的）

因此，解决方案是一个单独的课外声明：

std::ostream &operator<<(std::ostream &os,const Feature::tKeyboard &);

Answer 2

可能的原因是找不到::operator<<，因为未搜索全局命名空间。并且您不能将operator<<添加到::std::。如果是这种情况，可以使用本地using ::operator<<进行修复。

Answer 3

您是否尝试将输出运算符添加到命名空间std以帮助ADL？

namespace std{
ostream &operator<<(ostream &os,const Feature::tKeyboard &k)
{
  os <<" Keyboard >\n";
  for (Feature::cKeysIter i=k.begin() ; i!= k.end() ; i++)
  {
    os << "key name:"<<(*i).first << " ="<< (*i).second ;
  }
  return os;
}
}

重载＆lt;＆lt;地图的操作员

3 个答案: