21. Caution: when operator overloading can hurt your users

Consider:

#include <iostream>

class BoolFlag { 
    bool m_flag;
public:
    BoolFlag( bool flag )
    : m_flag( flag )
    {
    }

    bool operator|| ( BoolFlag const& other )
    {
        return m_flag || other.m_flag;
    }
};

BoolFlag download( char const* URL )
{
    std::cout << "Dowloading " << URL << '\n';
    //...
    return BoolFlag( true );
}

int main( )
{
    char const* URL = "www.xyz.com/lib/file.xml";
    BoolFlag download_complete = download( URL );

    if ( download_complete || download( URL ) ) {
        // process file...
    }
    return 0;
}

Do not overload operators

    || and &&

Clients will expect usual semantics...

...and you can't provide them!