cmkr/include/enum_helper.hpp

// https://codereview.stackexchange.com/a/14315

#include <algorithm>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>

// This is the type that will hold all the strings.
// Each enumeration type will declare its own specialization.
// Any enum that does not have a specialization will generate a compiler error
// indicating that there is no definition of this variable (as there should be
// be no definition of a generic version).
template <typename T>
struct enumStrings {
    static std::vector<std::string> data;
};

// This is a utility type.
// Created automatically. Should not be used directly.
template <typename T>
struct enumRefHolder {
    T &enumVal;
    enumRefHolder(T &enumVal) : enumVal(enumVal) {}
};
template <typename T>
struct enumConstRefHolder {
    T const &enumVal;
    enumConstRefHolder(T const &enumVal) : enumVal(enumVal) {}
};

// The next two functions do the actual work of reading/writing an
// enum as a string.
template <typename T>
std::ostream &operator<<(std::ostream &str, enumConstRefHolder<T> const &data) {
    return str << enumStrings<T>::data[data.enumVal];
}

template <typename T>
std::istream &operator>>(std::istream &str, enumRefHolder<T> const &data) {
    std::string value;
    str >> value;

    // These two can be made easier to read in C++11
    // using std::begin() and std::end()
    //
    auto begin = std::begin(enumStrings<T>::data);
    auto end = std::end(enumStrings<T>::data);

    auto find = std::find(begin, end, value);
    if (find != end) {
        data.enumVal = static_cast<T>(std::distance(begin, find));
    } else {
        throw std::invalid_argument("");
    }
    return str;
}

// This is the public interface:
// use the ability of function to deduce their template type without
// being explicitly told to create the correct type of enumRefHolder<T>
template <typename T>
enumConstRefHolder<T> enumToString(T const &e) {
    return enumConstRefHolder<T>(e);
}

template <typename T>
enumRefHolder<T> enumFromString(T &e) {
    return enumRefHolder<T>(e);
}