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 intoeColors
.
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>();
}
最佳答案
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 )。
关于C++11 初始化列表、数组和枚举的乐趣,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/8606315/