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operator<< overloading

Posted on 2003-11-16
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Last Modified: 2010-04-01
I have two classes that I need to overload the stream operator << on. One works, the other doesn't, but I'm having trouble seeing why the latter doesn't work.

Here they are. I'm hoping someone can help me point out the difference.

/////////////////////////////////////////////////////////////////////////////
// this class overloads << just fine.
class Location {
public:
    Location(Country country, std::string& location_name);
    Country country() const { return myCountry; }
    const std::string& locationName() const { return myLocationName; }
    float longDistanceRateFromCanada() const;

private:
    Country myCountry;
    std::string myLocationName;
};

static std::ostream& operator<<(std::ostream& os, const Location& location) {
    os << location.locationName();
    return os;
}
////////////////////////////////////////////////////////////////
// this class does not

class PhoneNumber {
private:
    unsigned long myPhoneNumber;
public:
    PhoneNumber(unsigned long p) { myPhoneNumber = p; }
    int areaCode();
    unsigned long getNumber();
};

static std::ostream& operator<<(std::ostream& os, const PhoneNumber& pn) {
    os << pn.getNumber();
    return os;
}
///////////////////////////////////////////////////////////

Thanks for any help you can provide.




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Question by:seagu11
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Expert Comment

by:AlexFM
ID: 9758488
What doesn't work exactly? Not compiled, not linked, doesn't print anything, prints junk etc.
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Accepted Solution

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AlexFM earned 50 total points
ID: 9758490
My guess is: make getNumber function const:

unsigned long getNumber() const;

You can also write it's implementation.
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Author Comment

by:seagu11
ID: 9758548
I apologize for not including the error. I totally meant to.

In file included from PhoneNumber.cpp:1:
PhoneNumber.h: In function `class ostream & operator <<(ostream &, const PhoneNumber &)':
PhoneNumber.h:16: passing `const PhoneNumber' as `this' argument of `long unsigned int PhoneNumber::getNumber()' discards qualifiers
*** Error code 1
0
 

Author Comment

by:seagu11
ID: 9758554
Thanks AlexFM. Making the getNumber() function const did the trick.
it's implementation was simply

unsigned long PhoneNumber::getNumber() const{
    return myPhoneNumber;
}

Now I need to find out *why* that worked so I don't do it again.
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LVL 48

Expert Comment

by:AlexFM
ID: 9758581
static std::ostream& operator<<(std::ostream& os, const PhoneNumber& pn)

This declares pn as const, so you cannot call non-const PhoneNumber members. Marking getNumber const (and it is really const because it doesn't change class members) makes C++ compiler happy.
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