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compiling error, tried debugging but no luck

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compiling error, tried debugging but no luck

 
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  #1
Apr 13th, 2005
alright i am trying to write a program that implements a class AirborneLocation that stores info on planes in the sky yada, yada, yada....i won't post the whole src code cuz it is hella long but i will post the section that gives me the compiling error

C Syntax (Toggle Plain Text) 
  1. ostream& operator<< (ostream& os, const AirborneLocation& plane){
  2. 	os << plane.name << ":" << endl;
  3. 	os << "\tPlane's ID: " << plane.ID << endl;
  4. 	os << "\tPlane's Class: ";
  5. 	if (plane.plane_class == 'C')
  6. 		os << "Civilian";
  7. 	else
  8. 		os << "Military";
  9. 	os << endl;
  10. 	os << "\tPlane's Location: ";
  11. 	if (plane.coord_x < 0.0)
  12. 		os << abs(plane.coord_x) << " km west, ";
  13. 	else
  14. 		os << plane.coord_x << " km east, ";
  15. 	if (plane.coord_y < 0.0)
  16. 		os << abs(plane.coord_y) << " km south, ";
  17. 	else
  18. 		os << plane.coord_y << " km north, ";
  19. 	if (plane.coord_z < 0.0)
  20. 		os << abs(plane.coord_z) << " km down";
  21. 	else
  22. 		os << plane.coord_z << " km up";
  23. 	os << endl;
  24. 	os << "\tDistance to plane: " << plane.distance();
  25. }

it gives me the error:

prog7.cpp: In function `std::ostream& operator<<(std::ostream&, const
AirborneLocation&)':
prog7.cpp:117: error: passing `const AirborneLocation' as `this' argument of `
double AirborneLocation::distance()' discards qualifiers

line 117 in the program is this line:

C Syntax (Toggle Plain Text) 
  1. os << "\tDistance to plane: " << plane.distance();

whats goin on here??
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Re: compiling error, tried debugging but no luck

 
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  #2
Apr 13th, 2005 
ostream& operator<< (ostream& os, const AirborneLocation& plane)
os << "\tDistance to plane: " << plane.distance();
Does plane.distance() return an AirborneLocation? Or should it be called like this?
C Syntax (Toggle Plain Text) 
  1. os << "\tDistance to plane: " << plane;
And shouldn't operator<< return os?
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Re: compiling error, tried debugging but no luck

 
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  #3
Apr 13th, 2005
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Originally Posted by Dave Sinkula 
ostream& operator<< (ostream& os, const AirborneLocation& plane)
os << "\tDistance to plane: " << plane.distance();
Does plane.distance() return an AirborneLocation? Or should it be called like this?
C Syntax (Toggle Plain Text) 
  1. os << "\tDistance to plane: " << plane;
And shouldn't operator<< return os?

you're right i forgot the "return os;" statement....and no...plane.distance() is defined as

C Syntax (Toggle Plain Text) 
  1. double AirborneLocation :: distance(){
  2.           return sqrt(coord_x*coord_x + coord_y*coord_y + coord_z*coord_z);
  3. }
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Re: compiling error, tried debugging but no luck

 
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  #4
Apr 13th, 2005
Make this change and call it good:
double AirborneLocation :: distance() const {
I'm here to prove you wrong.
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Re: compiling error, tried debugging but no luck

 
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Apr 13th, 2005
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Originally Posted by Narue
Make this change and call it good:
double AirborneLocation :: distance() const {
why would i need the const there?
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Re: compiling error, tried debugging but no luck

 
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  #6
Apr 13th, 2005
>why would i need the const there?
The member function doesn't change any data members, so you can safely say that it won't do so by qualifying it as const. Now you can call the member function on behalf of both const and non-const objects. The former is not possible right now because the compiler can't guarantee that the member function will not modify data members if it isn't qualified as const.

Another option is to have your overloaded operator<< take a AirborneLocation& instead of a const AirborneLocation&. However, this opens up the potential for other issues that you really don't want to deal with. Qualifying any member functions that don't modify the object's state is a good habit to get into.
I'm here to prove you wrong.
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