c++ - Initializing fixed number of variables with an initializer list -

i want initialize object initializer list. problem is, initializer list able contain unpredictable number of elements, need initialize variables. user may send number of list elements, , need 4 of them.

i wrote following code, looks long , inefficient me. there better way of doing this?

pixel::pixel(std::initializer_list<uint8_t> channels) {     switch (channels.size())     {         case 0:             r = 0;             g = 0;             b = 0;             = 0;             break;         case 1:             r = *(channels.begin() + 0);             g = 0;             b = 0;             = 0;             break;         case 2:             r = *(channels.begin() + 0);             g = *(channels.begin() + 1);             b = 0;             = 0;             break;         case 3:             r = *(channels.begin() + 0);             g = *(channels.begin() + 1);             b = *(channels.begin() + 2);             = 0;             break;         default:             r = *(channels.begin() + 0);             g = *(channels.begin() + 1);             b = *(channels.begin() + 2);             = *(channels.begin() + 3);     } } 

(note: know can done passing r, g, b, values 4 separate arguments. main purpose learn how initializer list feature.)

the best come when using std::initialiser_list

struct pixel {     pixel(std::initializer_list<uint8_t> rgba)     : _rgba { rgba }     {         switch(_rgba.size()) {              case 0: _rgba.push_back(0);             case 1: _rgba.push_back(0);             case 2: _rgba.push_back(0);             case 3: _rgba.push_back(0);             case 4: break;             default:                 throw std::invalid_argument { "" };         }      }      std::vector<uint8_t> _rgba; }; 

... ...

probably correct way solve problem this:

struct pixel {     pixel(uint8_t r = 0, uint8_t g = 0, uint8_t b = 0, uint8_t = 0)     : r(r)     , g(g)     , b(b)     , a(a)     {}      uint8_t r,g,b,a; }; 

because

1. it fail compile if provide invalid arguments
2. it's optimally efficient
3. it's least surprising solution maintaining code
4. it automatically supports initializer_list syntax @ call site

examples:

int main() {     pixel p1 { 10, 20, 5, 255 };     pixel p2 { 10, 20, 5 };     pixel p3 { 10, 20 };     pixel p4 { 10 };     pixel p5 { };      pixel pv1 ( 10, 20, 5, 255 );     pixel pv2 ( 10, 20, 5 );     pixel pv3 ( 10, 20 );     pixel pv4 ( 10 );     pixel pv5;      return 0; }