c++ - 如何将枚举类写入文件并在 C++ 中访问它？

Question

我正在尝试将类对象的内容写入文件。该对象有一个枚举类成员，我无法使用 ofstream 将其写入文件。

我收到以下错误。

error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘EnumClass_scope::EnumCass_name’)

任何线索都会有所帮助。

Answer 1

另一种选择是<<为您的枚举手动重载运算符：

#include <iostream>

enum class ESomething
{
    Nothing,
    Something,
    Everything,
};

std::ostream& operator<<(std::ostream& os, const ESomething& other)
{
    switch (other)
    {
    case ESomething::Nothing:
        os << "Nothing";
        break;
    case ESomething::Something:
        os << "Something";
        break;
    case ESomething::Everything:
        os << "Everything";
        break;
    default:
        break;        
    }
    return os;
    // Alternatively: std::cout << std::underlying_type(other); return os;
}

int main()
{
    ESomething hello = ESomething::Something;
    std::cout << hello << '\n';

    return 0;
}

您可以在哪里准确决定在每个单独的情况下输出什么，或者像其他答案提到的那样简单地将其转换为基础类型并输出。这是你的选择。

Answer 2

One way is to cast the enum type to the underlaying type. This can be done with already defined std::underlying_type_tto get the internal representation of the enum.

    template < typename T>
auto toUnderlayingType( T t )
{
    return static_cast<std::underlying_type_t< T >>( t );
}

    template < typename T, typename U = std::underlying_type_t< T > >
void readToEnum( U u , T& t )
{
    t = static_cast< T >( u );
}

class Y
{
    public:

        enum class X
        {
            ONE,
            TWO
        } x;

        Y(): x{X::ONE} { }

        void Print()
        {
            std::cout << toUnderlayingType(x) << std::endl;
        }

        void Read()
        {
            std::underlying_type_t< X > tmp;
            std::cin >> tmp;
            readToEnum( x, tmp );
        }
};



int main()
{
    Y y;
    y.Print();
}

But in the case of serialization it is even better to use a better representation of content and data. As reflection is still not part of c++ you have to convert the enum value to any useful output and back. Maybe you will write text like "ONE" instead of 0 to the file. But all this is to broad for this answer. There are a long list of serializer libraries like boost and cerial

And you have to check if the incoming value is a valid one. Having the example above and assign a numerical 3 to it, the enum value is invalid. That topic is already discussed here: What happens if you static_cast invalid value to enum class?

Answer 3

在写入之前将变量转换为整数。读取时使用整数临时变量。

std::fstream stream;
enum class Enum { ... };

Enum variable;
stream << static_cast<int>(variable);

int temporary;
stream >> temporary;
variable = static_cast<Enum>(temporary);

如果您使用其他类型定义 enum int，请使用该类型。例如，使用enum class Enum : unsigned long long，使用static_cast<unsigned long long>。