重载ostream <<运算符

Question

I was wondering if it is possible to overload the << operator in the need for printing an object in the class in two certain way.

For example, I'm building a domino game so I need to print my cubes with the numbers : [1][6][6][3] And print the computers cubes : [ ][ ]

Answer 1

Here is one example of overloaded extraction and insertion operators:

/*
    Operator: <<
    This insertion operator outputs the data members of 
    class object instantiation in the format:(x,y)
    The returning type ostream&, allows 
    you to chain more then one objects to be printed.
*/
ostream& operator<< (ostream& os, class_name& object_name) {
   return os << '(' << object_name.get_data_memberx() 
             << ',' << object_name.get_data_membery()
             << ')';
}

/*
    Operator: >>
    This extraction operator creates an input stream it
    inserts to it values for of all the data members of
    the class passed by reference and returns it.
    Input format: (x,y)
*/
istream& operator>> (istream& is, class_name& object_name) {
     char par1, comma, par2;
     double x, y;

     is >> par1 >> x >> comma >> y >> par2;
     // check if any input
     if(!is) return is;
     // check for valid input format
     if (par1 != '(' || comma != ',' || par2 != ')') {
         // set failbit to indicate invalid input format
         is.clear(ios_base::failbit);
         return is;
     }
     // assign input values to second argument
     object_name = class_name(x,y);
     return is;
}

You could use the above example and modify the formats to match the desired result.

Answer 2

While the others have answered how to overload operator<< , it's not clear how to have two different formats. I recommend this: temporary objects.

struct showdomino {
    domino& d; 
};
std::ostream& operator<<(std::ostream& out, showdomino dom) 
{return out << '[' << dom->d.number << ']';}

struct hidedomino {};
std::ostream& operator<<(std::ostream& out, hidedomino dom) 
{return out << "[ ]";}

Then usage is vaguely like this:

if (show_it)
    std::cout << showdomino{d} << '\n';
else
    std::cout << hidedomino{} << '\n';

Answer 3

The << operator used for cout is actually the bitwise left shift operator that is overloaded to obtain this behaviour.

I don't really understand what you want to achieve exactly (a code snippet would have been helpful). But, it seems you want to overload << for an ostream and a YourClass type. You can have something like:

void operator<<(ostream str, YourClass cls) {
    // generate required output
}

Note that this acts as a regular function, just the calling syntax is different.