Object as array index

Question

I have the following class:

class MyInteger
{
private:
    __int64 numero;

    static __int64 int64Pow(__int64, __int64);
public:
    // It doesn't matter how these methods are implemented
    friend class MyInteger;
    MyInteger(void);
    MyInteger(const MyInteger&);
    MyInteger(const __int64&);
    ~MyInteger(void);

    static MyInteger const minValue;
    static MyInteger const maxValue;

    MyInteger& operator = (const MyInteger&);
    MyInteger operator + (const MyInteger&) const;
    MyInteger operator - (const MyInteger&) const;
    MyInteger operator * (const MyInteger&) const;
    MyInteger operator / (const MyInteger&) const;
    MyInteger& operator += (const MyInteger&);
    MyInteger& operator -= (const MyInteger&);
    MyInteger& operator *= (const MyInteger&);
    MyInteger& operator /= (const MyInteger&);
    MyInteger operator % (const MyInteger&) const;
    MyInteger& operator %= (const MyInteger&);
    MyInteger& operator ++ ();
    MyInteger operator ++ (int);
    MyInteger& operator -- ();
    MyInteger operator -- (int);
    bool operator == (const MyInteger&) const;
    bool operator != (const MyInteger&) const;
    bool operator > (const MyInteger&) const;
    bool operator < (const MyInteger&) const;
    bool operator >= (const MyInteger&) const;
    bool operator <= (const MyInteger&) const;

    int toStdInt() const
    {
        return (int)numero;
    }
    float toStdFloat() const;
    double toStdDouble() const;
    char toStdChar() const;
    short toStdShortInt() const;
    long toStdLong() const;
    long long toStdLongLong() const;
    unsigned int toStdUInt() const;
    __int64 toStdInt64() const;
    unsigned __int64 toStdUInt64() const;
    unsigned long long int toStdULongLong() const;
    long double toStdULongDouble() const;

    template<class Type>
    Type& operator[](Type* sz)
    {
        return sz[toStdULongLong()];
    }
};

template<class Type>
Type* operator+(const Type* o1, const MyInteger& o2)
{
    return ((o1) + (o2.toStdInt()));
}

I'd like to use this class to access array elements like this:

MyInteger myInt(1);
int* intPtr = (int*)malloc(sizeof(int) * N);
intPtr[myInt] = 1;

I thought that the function

template<class Type>
Type* operator+(const Type* o1, const MyInteger& o2)
{
    return ((o1) + (o2.toStdInt()));
}

could solve my problem, because as this post reports (Type of array index in C++) "The expression E1[E2] is identical (by definition) to *((E1)+(E2))", but I get the C2677 error ('[' operator: no global operator found which takes type 'MyInteger' (or there is no acceptable conversion))

Can someone clarify me this situation? Thanks

@JoachimPileborg Thanks for the answer, but it isn't what I'm looking for — Matteo Umili
– Matteo Umili, Commented Sep 12, 2013 at 13:34

Lake · Accepted Answer · 2013-09-12 12:54:17Z

3

You may be able to do that by overriding the cast to int of your MyInteger class in a way similar to:

class MyInteger {
   ...

   operator int() const
   {
       return toStdInt(); /** Your class as an array index (int) */
   }

   ...
}

answered Sep 12, 2013 at 12:54

Lake

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6 Comments

Matteo Umili Over a year ago

I already tried, but when I do something like MyInteger i(0); i + 1; I get error C2666: 'MyInteger::operator +': 2 overloads have similar conversions

Lake Over a year ago

Actually I don't think you have to override all the mathematical operators, you could just override the cast to integer and binary operators should work automatically.. I might be wrong though.

Pete Becker Over a year ago

This is not a cast. It's a conversion operator. A cast is something you write in your source code to tell the compiler to do a conversion.

Jarod42 Over a year ago

i + 1 may be seen as i + MyInterger(1) or static_cast<int>(i) + 0. You may solve it by Adding explicit to your "int constructor" to forbid the implicit i + MyInterger(1) or overload all binary operator...

Matteo Umili Over a year ago

@Lake It seems to work, thank you. Can someone explain me also why the method I tried didn't work?

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