在枚举类上使用std :: get时需要static_cast

Question

According to the c++ reference , the template parameter of std::get is a std::size_t . Why does one need an explicit cast when such a parameter is an enum class with base type std::size_t , instead of having an implicit conversion?

See following example

#include <tuple>

enum class labels : std::size_t { red, green, blue };

int main()
{
  std::tuple<int, int, double> a;
  // std::get<labels::red>(a) = 0;
  std::get<static_cast<std::size_t>(labels::red)>(a) = 0;
  std::get<static_cast<std::size_t>(labels::green)>(a) = 0;
  std::get<static_cast<std::size_t>(labels::blue)>(a) = 0;

  return 0;
}

Uncommenting the line gives rise to a compilation error (gcc 7.3.0)

tuple.cpp:8:26: error: could not convert template argument ‘red’ from ‘labels’ to ‘long unsigned int’
   std::get<labels::red>(a) = 0;

Answer 1

That's because enum class (as opposed to simple enum ) is not implicitely convertible to numeral type. You need to explicitely cast it to obtain the numeric value, for example using static_cast<>() .