Question

我在c ++遇到问题：

#include <iostream>
 class Apple{
public:
    int price = 100;
    enum class color {
    red =1, green, yellow
    };



};
int main() {
 Apple apple;
 std::cout << Apple::color::green << std::endl;

}

当我尝试编译此代码时，会出现以下消息：

[错误]＆＃39; Apple :: color＆＃39;不是类或命名空间

Answer 1

看起来您正在使用预C ++ 11编译器或c ++ 11标志未启用。

使用正确的c ++ 11标志后，您必须重载operator <<

friend std::ostream& operator <<( std::ostream& os, const color& c )
{
  /* ... */
  return os;
}

Answer 2

启用c++11因为enum class是一个c ++ 11功能，起诉 - std=c++11编译器标志。
如果您想要<< cout

Apple::color

以下内容应该有效：

#include <iostream>

class Apple {
 public:
  int price = 100;
  enum class color { red = 1, green, yellow };
};

std::ostream& operator<<(std::ostream& os, const Apple::color& c) {
  if (c == Apple::color::red) std::cout << "Red\n";
  if (c == Apple::color::green) std::cout << "Green\n";
  if (c == Apple::color::yellow) std::cout << "Yellow\n";
  return os;
}

int main() {
  Apple apple;
  std::cout << Apple::color::green << std::endl;
}

Answer 3

为了使用enum class，您的编译器必须支持C ++ 11。例如，如果您使用clang或g ++构建命令，可以通过附加-std=c++11来实现。新版本的Visual Studio自动启用C ++ 11。

正如@ no operator "<<所指出的那样，你应该得到的错误是enum class，因为let startHour: Int = 6 let endHour: Int = 23 let date1: NSDate = NSDate() let gregorian: NSCalendar = NSCalendar(calendarIdentifier: NSCalendar.Identifier.gregorian)! let components: NSDateComponents = gregorian.components(([.day, .month, .year]), from: date1 as Date) as NSDateComponents components.hour = startHour components.minute = 0 components.second = 0 let startDate: NSDate = gregorian.date(from: components as DateComponents)! as NSDate components.hour = endHour components.minute = 0 components.second = 0 let endDate: NSDate = gregorian.date(from: components as DateComponents)! as NSDate timePicker.datePickerMode = .time timePicker.minimumDate = startDate as Date timePicker.maximumDate = endDate as Date timePicker.setDate(startDate as Date, animated: true) timePicker.reloadInputViews() timePicker.frame = CGRect(x: 0.0, y: (self.view.frame.height/2 + 60), width: self.view.frame.width, height: 150.0) timePicker.backgroundColor = UIColor.white // timePicker.reloadInputViews() self.view.addSubview(timePicker) timePicker.addTarget(self, action: #selector(RoundTripViewController.startTimeDiveChanged), for: UIControlEvents.valueChanged)没有隐式转换为任何内容，因此不会输出整数值。

Answer 4

P0W的答案在两个计数上都是正确的，但是如果您只想输出基础值，那么投射可能更简单，而不是使插入运算符过载。

using enum_type = std::underlying_type<Apple::color>::type;
enum_type value = (enum_type)Apple::color::green;
std::cout << value << '\n';

C ++如何访问内部枚举类？

4 个答案: