Using class as enum-like, or with a specific value

Question

I would like my function to accept a compression-level (which can take values 0..9). I want the user to be able to set the level manually Compression(7) or use some defaults like Compression::High, Compression::Medium.

I thought of the following:

class Compression {
public:
    Compression(size_t level) : m_level(level) {};
    static const Compression Medium;
    static const Compression High;
    size_t get() const { return m_level; };
private:
    size_t m_level;
};

const Compression Compression::High{9};
const Compression Compression::Medium{5};

The problem is that I am developing a header-only library, and doing this leads to "multiple definitions of 'Compression::High' and 'Compression::Medium". How can this be solved?

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Edit

Challenges:

• I only want to have one function overload foo(Compression) that acts on the Compression option (foo(size_t) is already in use for something else).
• I'm stuck to C++11 (maybe C++14, but not higher).

Answer 1

GCC version 9.3.0 accepts that code (C++17):

inline const Compression Compression::High{9};
inline const Compression Compression::Medium{5};

Please note the inline word.