Re: enum class + unordered_map problem

=?UTF-8?B?RGFuaWVsIEtyw7xnbGVy?= <>
Wed, 16 Mar 2011 01:46:32 CST
On 2011-03-16 03:34, Helmut Jarausch wrote:


given enum class Color { red=1, green= 2, blue= 4, black=8 };

I've used a std::unordered_map<std::string,Color> with success (using
gcc-4.6). That example uses the new initializer syntax, as well.

But the following example fails (what am I missing, or is it a gcc bug?)

using std::string;

enum class Color { red=1, green= 2, blue= 4, black=8 };

std::unordered_map<Color,string> ColorMap {
    {Color::red,"red"}, {Color::green,"green"},
    {Color::blue,"blue"}, {Color::black,"black"}

int main() {
  std::cout<< ColorMap[Color::green]<< "\n";
  return 0;
// during instantion I get a very long error message ending in
// ... undefined reference to `std::hash<Color>::operator()(Color) const'

This is no compiler bug nor library bug. You just miss that the class
template hash has no specified primary template. The library just
requires the provision of the following specializations:

1) Header <functional>:

template <> struct hash<bool>;
template <> struct hash<char>;
template <> struct hash<signed char>;
template <> struct hash<unsigned char>;
template <> struct hash<char16_t>;
template <> struct hash<char32_t>;
template <> struct hash<wchar_t>;
template <> struct hash<short>;
template <> struct hash<unsigned short>;
template <> struct hash<int>;
template <> struct hash<unsigned int>;
template <> struct hash<long>;
template <> struct hash<long long>;
template <> struct hash<unsigned long>;
template <> struct hash<unsigned long long>;
template <> struct hash<float>;
template <> struct hash<double>;
template <> struct hash<long double>;
template<class T> struct hash<T*>;

2) Header <system_error>:

template <> struct hash<error_code>;

3) Header <bitset>:

template <size_t N> struct hash<bitset<N>>;

4) Header <memory>:

template <class T, class D> struct hash<unique_ptr<T, D>>;
template <class T> struct hash<shared_ptr<T>>;

5) Header <typeindex>:

template<> struct hash<type_index>;

6) Header <string>:

template <> struct hash<string>;
template <> struct hash<u16string>;
template <> struct hash<u32string>;
template <> struct hash<wstring>;

7) Header <vector>:

template <class Allocator> struct hash<vector<bool, Allocator>>;

8) Header <thread>:

template <> struct hash<thread::id>;

There are specializations defined for any form of enumeration type (not
even for unscoped enums).

HTH & Greetings from Bremen,

Daniel Kr??gler

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