Input from stream to enum type

Question

How to input from stream to enum type?

I can do it so

unsigned int sex = 0;
stream >> sex;
student.m_bio.sex = static_cast<Sex>(sex);

Otherwise?

Answer 1

inline std::istream & operator>>(std::istream & str, Sex & v) {
  unsigned int sex = 0;
  if (str >> sex)
    v = static_cast<Sex>(sex);
  return str;
}

If you want to ensure that the value is valid, you can do something like this:

enum Sex {
    Male,
    Female,
    Sex_COUNT
};

inline std::istream & operator>>(std::istream & str, Sex & v) {
  unsigned int sex = 0;
  if (!(str >> sex))
    return str;
  if (sex >= Sex_COUNT) {
    str.setstate(str.rdstate() | std::ios::failbit);
    return str;
  }
  v = static_cast<Sex>(sex);
  return str;
}

Answer 2

This question was asked in a more general form over here: How to read enums from a std::istream in a generic fashion. The OP almost had a working solution as it was; he just had some trouble with const, and a few unnecessary angle-brackets. Here's the working solution fleshed out:

#include <iostream>
#include <type_traits>

enum enSide { eLeft, eRight };
enum enType { eConUndefined, eConRoom };

template<typename Enum>
class EnumReader
{
    Enum& e_;

    friend std::istream& operator>>(std::istream& in, const EnumReader& val) {
      typename std::underlying_type<Enum>::type asInt;
      if (in >> asInt) val.e_ = static_cast<Enum>(asInt);
      return in;
    }
  public:
    EnumReader(Enum& e) : e_(e) {}
};

template<typename Enum>
EnumReader<Enum> read_enum(Enum& e)
{
    return EnumReader<Enum>(e);
}

class MyClass {
    enSide mSide;
    enType mType;
    int mTargetId;
  public:
    friend std::istream& operator>>(std::istream& in, MyClass& val) {
        in >> read_enum(val.mSide) >> read_enum(val.mType) >> val.mTargetId;
        return in;
    }
};

The read_enum function template serves the same purpose here as std::make_pair or std::make_shared in the standard library: it lets us dispense with the angle brackets. You could equally well write in >> EnumReader<enSide>(val.mSide) >> EnumReader<enType>(val.mType), but that's more typing (pun intended).

A few vendors' standard libraries are allegedly still missing std::underlying_type from their <type_traits> headers. If you have one of these incomplete libraries, you can use one of the workarounds listed at How to know underlying type of class enum?.

Answer 3

This is not pretty but should do it

stream >>  reinterpret_cast<std::underlying_type<Sex>::type &>(student.m_bio.sex);

Cheers, CC