重载<<操作符，用于std :: ostream

Question

ostream& operator <<(ostream& osObject, const storageRentals& rentals)
{
    osObject << rentals.summaryReport();
    return osObject;
}

summaryReport() is a void function, and it is giving me an error:

no operator "<<" matches these operands

but the error is not there if I change the summaryReport function to an int , but the problem I have with that is you have to return a value, and it is printing it out on the screen.

void storageRentals::summaryReport() const
{
   for (int count = 0; count < 8; count++)
      cout << "Unit: " << count + 1 << "    " << stoUnits[count] << endl;
}

Is there any way to overload cout << with a void function?

Answer 1

You should define summartReport taking ostream& as parameter, as shown here:

std::ostream&  storageRentals::summaryReport(std::ostream & out) const
{
    //use out instead of cout
    for (int count = 0; count < 8; count++)
        out << "Unit: " << count + 1 << "    " << stoUnits[count] << endl;

    return out; //return so that op<< can be just one line!
}

then call it as:

ostream& operator <<(ostream& osObject, const storageRentals& rentals)
{
    return rentals.summaryReport(osObject); //just one line!
}

By the way, it is not called "overloading cout" . You should say, "overloading operator<< for std::ostream .

Answer 2

There are two things you need to do here. Make storageRentals::summaryReport() take an std::ostream& (you can default this to std::cout ):

void storageRentals::summaryReport(std::ostream& os) const
{
    for (int count = 0; count < 8; count++)
    {
        os << "Unit: " << count + 1 << "    " << stoUnits[count] << endl;
    }
}

Then call it thus:

ostream& operator <<(ostream& osObject, const storageRentals& rentals)
{
    rentals.summaryReport(osObject);
    return osObject;
}

---

Note: An advantage of making storageRentals::summaryReport() take an std::ostream& is that you can pass it an std::ostringstream in your unit-tests and assert that it is providing correct output.

Answer 3

If you overload cout , you may make some other place using code hard to understand, or more confusing. In fact, you could make your own class to complete the work. eg make a class class MyPring , and overload its operator << .

Answer 4

Your storage report implicitely is always streamed to cout. Imagine someone calling your function this way and having the rentals on cout instead of the file.

fstream fs("out.txt");
storageRentals rentals;
fs << rentals;

Why don't you stream your class like this:

ostream& operator <<(ostream& osObject, const storageRentals& rentals)
{

  for (int count = 0; count < 8; count++) {
      osObject << "Unit: " << count + 1 << "    " << rentals.stoUnits[count] << endl;
  }
  return osObject;
}

If the stoUnits member is private you need to make the stream function a friend of your storage class.

friend ostream& operator<<(ostream& osObject, const storageRentals& rentals);