How to overload the << operator so it only affects a member of a class?

Question

I have a class Bitset that stores a vector of chars, and I want to be able, whenever I use cout << char, to cast that char into a short int ONLY if it is part of that class.

Code:

template<long long X>
class Bitset
{
    public: std::vector<unsigned char> bit = std::vector<unsigned char> ((X+7)/8);

    public:
        /* constructors */

        friend std::ostream &operator<< (std::ostream &output, const char x);
};

std::ostream &operator<< (std::ostream &output, const char x)
{
    output<<(short)(x);
    return output;
}

The idea is that if I write:

Bitset a;
/* code */
cout << a.bit[x];
cout << 'a';

I want to cast a.bit[x] into a short, but not 'a' as well.

It's a bit unclear what you're asking. When you say "I want to be able, whenever I use cout<<char, to cast that char into a short int ONLY if it is part of that class", it sounds like what you're trying to achieve is completely unrelated to the Bitset class. Do you mean that you want cout << a.bit[x]; to always print a number instead of an ASCII character? This will need something other than char, but you could do it with a simple struct that wraps a char. You could also use std::byte to more clearly document that this is not a character — alter igel

Remy Lebeau Remy Lebeau · Accepted Answer · 2020-06-23T21:53:32

You can't overload operator<< for char to behave the way you want. It has no idea where the char comes from, so it can't behave differently based on the source.

To make this work the way you want, you will have to make Bitset implement its own operator[] that returns a proxy object, and then you can overload operator<< for that proxy, eg:

template<long long X>
class Bitset
{
private:
    std::vector<unsigned char> bits = std::vector<unsigned char> ((X+7)/8);

public:
    /* constructors */

    class BitProxy
    {
    private:
        unsigned char &bit;

    public:
        BitProxy(unsigned char &bit) : bit(bit) {}

        BitProxy& operator=(unsigned char x) { bit = x; return *this; }
        operator unsigned char() const { return bit; }
    };

    BitProxy operator[](size_t index) { return BitProxy(bits[index]); }

    friend std::ostream& operator<< (std::ostream &output, const BitProxy &x)
    {
        output << static_cast<short>(static_cast<unsigned char>(x));
        return output;
    }
};

Bitset a;
// populate a as needed...
cout << a[x];
cout << 'a';

Live Demo

How to overload the << operator so it only affects a member of a class?

2 Answers