CODEWITHSUNDEEP
c++mfc
Cheok Yan Cheng
Cheok Yan Cheng

Reputation: 42760

Object as key in CMap

I have a class named error_code. I use it as key for std::map and CMap (MFC). I am able to make it work for std::map, but not CMap. May I know how I can do so?

// OK!
std::map<error_code, int> m;
m[error_code(123)] = 888;

// error C2440: 'type cast' : cannot convert from 'error_code' to 'DWORD_PTR'
CMap <error_code, error_code&, int, int& > m; 
m[error_code(123)] = 888;

class error_code {
public:
    error_code() : hi(0), lo(0) {}
    error_code(unsigned __int64 lo) : hi(0), lo(lo) {}
    error_code(unsigned __int64 hi, unsigned __int64 lo) : hi(hi), lo(lo) {}

    error_code& operator|=(const error_code &e) {
        this->hi |= e.hi;
        this->lo |= e.lo;
        return *this;
    }

    error_code& operator&=(const error_code &e) {
        this->hi &= e.hi;
        this->lo &= e.lo;
        return *this;
    }

    bool operator==(const error_code& e) const {
        return hi == e.hi && lo == e.lo;
    }

    bool operator!=(const error_code& e) const {
        return hi != e.hi || lo != e.lo;
    }

    bool operator<(const error_code& e) const {
        if (hi == e.hi) {
            return lo < e.lo;
        }
        return hi < e.hi;
    }

    unsigned __int64 hi;
    unsigned __int64 lo;
};

Upvotes: 4

Views: 1942

Answers (1)

YeenFei
YeenFei

Reputation: 3218

A quick trace shows that template function belows is causing the error :

template<class ARG_KEY>
AFX_INLINE UINT AFXAPI HashKey(ARG_KEY key)
{
    // default identity hash - works for most primitive values
    return (DWORD)(((DWORD_PTR)key)>>4);
}

A quick fix would involve adding implicit conversion function to the user-defined type. I'm not sure what data will be stored so just randomly pick some attribute to form the required data.

class error_code {

    ...

    operator DWORD_PTR() const
    {
        return hi;
    }

    ...
}

Upvotes: 2

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