C++11 初始化列表、数组和枚举的乐趣

Question

背景

C++11 初始化列表可用于初始化 vector 和数组，并将参数传递给构造函数。

下面有一段代码，我想用 eCOLORS 从 eCOLORS::First 到 的所有枚举来初始化这样一个数组>eCOLORS::Last 使用初始化列表。

原始代码

由于所有信息在编译时都是已知的，所以我认为有办法解决这个问题。

enum class eCOLORS 
{
    kBLUE=0, kGREEN, kRED, kPURPLE,
        First=kBLUE, Last=kPURPLE 
};

template< typename E >
size_t Size()
{
    return (size_t)(E::Last) - (size_t)(E::First) + 1;
}

struct Foo
{
    Foo( eCOLORS color ) { }
};

int main(int argc, char** argv)
{
    Foo a[2] = {
        { eCOLORS::kRED   },
        { eCOLORS::kGREEN }
    };  // works, but requires manual maintenance if I add another color

    /* how to feed v with all the enums from First to Last
       without hard-coding?
    Foo v[Size<eCOLORS>()] = {

    };
    */
}

丑陋的伪答案

共识似乎是目前没有办法做到这一点。

My original intent in asking this question is, I want to automagically create an array of Foo objects whose initialization is solely based on the enumeration of eColors. I wanted a no maintenance solution that would work even after you add more entries into eColors.

使用 this earlier post 中的 Enum 类，我可以编写一个函数模板来提供我需要的功能。即使不使用该 Enum 类，您仍然可以从 eCOLORS::First 循环到 eCOLORS::Last，以及一些丑陋的转换。

我丑陋的伪答案很笨拙(没有编译时初始化列表好)，但至少它是零维护。

注意:如果出现更好的解决方案，我会相应地更新 OP。

template <typename T, typename E>
std::vector< T >
Initialize_With_Enums()
{
  std::vector< T > v;
  for( auto p : Enum<E>() )
    v.push_back( T( p ));
  return v;
}

int main( int argc, char** argv )
{
  auto w = Initialize_With_Enum<Foo,eCOLORS>();
}

Answer 1

您可以使用可变参数模板以及我将称之为“索引技巧”的方法来做到这一点。

typedef std::underlying_type<eCOLORS>::type underlying;

// just a type to carry around variadic pack of numbers
template <underlying...> struct indices {};

// A template to build up a pack of Count numbers from first
// third parameter is an accumulator
template <underlying First, underlying Count, typename Acc = indices<>>
struct make_indices;

// base case
template <underlying X, underlying... Acc>
struct make_indices<X, 0, indices<Acc...>> { typedef indices<Acc...> type; };
// recursive build up of the pack
template <underlying First, underlying Count, underlying... Acc>
struct make_indices<First, Count, indices<Acc...>>
    : make_indices<First, Count-1, indices<First+Count-1, Acc...>> {};

size_t const max_colors = underlying(eCOLORS::Last) - underlying(eCOLORS::First)+1;

// shortcut
typedef make_indices<
          underlying(eCOLORS::First),
          max_colors
        >::type all_eCOLORS_indices;

// takes a dummy parameter with the pack we built
template <underlying... Indices>
std::array<eCOLORS, max_colors> const& all_colors(indices<Indices...>) {
    // convert each number to the enum and stick it in an static array
    static std::array<eCOLORS, max_colors> const all = {
        eCOLORS(Indices)...
    };
    return all;
}

std::array<eCOLORS, max_colors> const& all_colors() {
    // create a dummy object of the indices pack type and pass it
    return all_colors(all_eCOLORS_indices());
}

这假设所有枚举器都是连续的，并且需要 GCC 4.6 不支持的 std::underlying_type(将在 4.7 中，但您可以 emulate it to a certain extent )。