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在将 boost::lexical cast 与具有重载输入/输出运算符的自定义类一起使用时，在 Max OSX 上编译时，我得到了一个模棱两可的重载运算符>>。在这种情况下，Boost 1.59 是使用 Macports 安装的。

下面是一个独立的示例，说明了错误和所需的结果：

#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;
}

为了说明所需的行为，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 >> 运算符进行适当的解析。

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

要改为使用 boost::lexical_cast，请使用“-D__DEMO__”进行编译，它会给出以下错误：

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__”都有效。任何想法/提示将不胜感激！

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1 回答 1

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我没有你的环境来精确分析。然而，看起来有一个候选库operator>>对 lexical_cast 实现传递的第一个参数有更好的匹配，但（自然）在BoolYN参数上的匹配更差。请特别注意，诊断中的操作数类型是basic_stream<wchar_t>，而您operator>>采用istream( basic_istream<char>，我相信)。因此，也许您可​​以通过将operator>>具有流字符类型的模板作为模板参数来解决此问题。

于 2015-09-04T18:25:04.090 回答