不明确的运算符>>在 Mac OSX 上使用 Boost 1.59 lexical_cast

Question

当使用boost::词法强制转换与自定义类重载输入/输出操作符时，我得到一个模糊的重载操作符>>在Max OSX上编译时。在本例中，Boost 1.59是使用Macports安装的。

下面是一个独立的示例，说明了错误和期望结果:

#include <iostream>
#include <string>
#include <sstream>
#ifdef __DEMO__
#include <boost/lexical_cast.hpp>
#endif
using namespace std;
enum BoolYN_ENUM { FALSE_YN, TRUE_YN, UNSET_YN};
class BoolYN{
public:
  BoolYN() : _dat(UNSET_YN){}
  BoolYN(const BoolYN& o) : _dat(o._dat) {}
  BoolYN& operator=(const BoolYN_ENUM& val){ _dat = val; return *this;}
  BoolYN& operator=(const BoolYN& o){ _dat = o._dat; return *this;}
  operator const char*() const{
    switch(_dat){
    case FALSE_YN:
      return "F";
    case TRUE_YN:
      return "T";
    default:
      return "U";
    }
  }
  operator int() const {return _dat;}
private:
  BoolYN_ENUM _dat;
};
istream& operator>>(istream& is, BoolYN& obj){
  string token;
  is >> token;
  if(token.size() > 0){
    char s = token[0];
    if(s == 'Y' || s == 'y'){
      obj = TRUE_YN;
    } else if (s == 'N' || s == 'n') {
      obj = FALSE_YN;
    } else {
      obj = UNSET_YN;
    }
  } else {
    obj = UNSET_YN;
  }
  return is;
}
ostream& operator<<(ostream& os, BoolYN& obj){
  os << (const char*) obj;
  return os;
}
int main(int argc, char** argv){
  for(int i=1; i<argc; i++){
    string argi(argv[i]);
#ifndef __DEMO__
    stringstream ss(argi);
    BoolYN boolval;
    ss >> boolval;
#else
    BoolYN boolval = boost::lexical_cast<BoolYN>(argi);
#endif
    cout << "Argument " << i << ": " << boolval << endl;
  }
  return 0;
}

要说明所需的行为，只需使用适当的包含路径进行编译。参数在命令行上给出，并使用重载的<<和>>操作符。

$ g++ main.cpp
$ a.out Yes No IDK
Argument 1: T
Argument 2: F
Argument 3: U

要使用boost::lexical_cast，使用"-D__DEMO__"编译，它应该给出下面的错误:

In file included from main.cpp:6:
In file included from /opt/local/include/boost/lexical_cast.hpp:32:
In file included from /opt/local/include/boost/lexical_cast/try_lexical_convert.hpp:35:
In file included from /opt/local/include/boost/lexical_cast/detail/converter_lexical.hpp:39:
In file included from /opt/local/include/boost/type_traits/has_right_shift.hpp:43:
/opt/local/include/boost/type_traits/detail/has_binary_operator.hpp:158:70: error: use of overloaded operator '>>' is ambiguous (with operand types 'std::__1::basic_istream<wchar_t>' and 'BoolYN')
   BOOST_STATIC_CONSTANT(bool, value = (sizeof(s_check(((make<Lhs>() BOOST_TT_TRAIT_OP make<Rhs>()),make<has_operator>())))==sizeof(::boost::type_traits::yes_type)));
... (very long error msg. truncated) ...

该代码在Linux, Boost v. 1.51上使用和不使用"-D__DEMO__"都可以工作。任何想法/提示将非常感激!

Answer 1

我没有你的环境来精确分析。然而，看起来有一个库候选operator>>与lexical_cast实现传递的第一个参数匹配得更好，但(自然地)与BoolYN参数匹配得更差。特别注意，诊断中的操作数类型是basic_stream<wchar_t>，而您的operator>>采用istream(我相信是basic_istream<char>)。所以也许你可以解决这个问题，使你的operator>>与流的字符类型的模板参数。