Operator overloading in C++ (operator-) [duplicate]

Question

I am working on a program to detect if one triangle is nested within another.

Here I created the struct point to define x and y as doubles.

struct Point { double x; double y; };

Here is where I implement the struct:

double eval (LineSegment line, Point p) { Point p1 = p - line.StartPoint; Point p2 = line.EndPoint - line.StartPoint; return p2.x * p2.y - p1.y * p2.x; }

When I compile it tells me, "no match for 'operator-' in 'p - line.LineSegment::StartPoint'." I don't really understand what is happening, I guess it doesn't understand which 'point' I want since i use them together?

I did research and found operator overloading, but it was sort of confusing to me. I don't really know how to implement Operator Overloading.

If someone could show me how exactly to go about doing this, it would be helpful.

Could you please show your LineSegment definition ? You need to overload - operator — P0W
– P0W, Commented Sep 11, 2013 at 18:20
You need to overload the - operator for Point if you want to use that syntax. — Carl Norum
– Carl Norum, Commented Sep 11, 2013 at 18:21
a. You aren't implementing a struct, you are implementing an eval function that goes something. — UpAndAdam
– UpAndAdam, Commented Sep 11, 2013 at 18:23

Community · Accepted Answer · 2017-05-23 10:31:54Z

It means that the compiler has no idea how to subtract Point objects. It knows how to subtract built-in types, but to use operators with custom types, you need to provide the behavior.

In this case you probably want something like:

struct Point { double x; double y; Point& operator-=( const Point& that ) { x -= that.x; y -= that.y; return this; } }; Point operator-(Point first, const Point& second) { return first -= second; }

For more guidance, see our C++-FAQ entry on Operator Overloading. Lots of good advice in there that will help you avoid some of the pitfalls, as well as good examples and recommendations how to maximize code re-use and readability (which really means maintainability).

In this case, you should also ask yourself whether you want to differentiate between absolute and relative coordinate pairs. If you do, then the difference between two points should be a Size or Offset, and not a Point. (If you've ever used a library with DateTime and TimeSpan classes, you've seen this concept in action. C++ itself uses it for the Standard Library's time_point vs duration, both found in namespace std::chrono. You have a two dimensional version of that here.)

+1 for nice code usage of an additional operator implemented effectively to simplify the non-member function work.
+1 for the last paragraph; in which case, you wouldn't want that operator-=, although you might want operator-=(Offset).
The answer is actually bad: You can only add/subtract a (math) vector to a point.

erenon · Accepted Answer · 2013-09-11 18:23:54Z

struct Point { double x; double y; Point operator-(const Point& that) const { Point result; result.x = this->x - that.x; result.y = this->y - that.y; return result; } };

Mike Seymour · Accepted Answer · 2013-09-11 18:24:39Z

If you want to subtract points, then you need to provide an overloaded operator to do it. C++ doesn't automatically generate arithmetic operations for class types, since most of the time they wouldn't make sense.

Something like this should do it:

Point operator-(Point lhs, Point rhs) { return {lhs.x-rhs.x, lhs.y-rhs.y}; }

although I'd probably introduce a new type (Vector or Offset or something) for the return type, to prevent the category error of using the offset between points when you should use a point.

Daniel Frey · Accepted Answer · 2013-09-11 18:23:37Z

Here's a basic sketch to get you started:

Point operator-( const Point& lhs, const Point& rhs ) { Point result; // make sure this is what you want: result.x = lhs.x - rhs.x; result.y = lhs.y - rhs.y; return result; }

Note this is a free function, it doesn't go inside of Point's definition. Just put it afterwards.

E. Moffat · Accepted Answer · 2013-09-11 18:24:44Z

Take a look at http://www.cplusplus.com/doc/tutorial/classes2/. It has a pretty good explanation on operator overloading.

What your code is saying is to subtract line.StartPoint from p. However, your struct doesn't have a defined behavior for that operation. So you can't use the syntax p - line.StartPoint unless you tell the compiler how to perform that operation with your struct.

The method signature in your struct will probably look something like this:

Point operator-(const Point &other) { }

You need to add the logic to that method that tells the compiler how to perform a subtraction operation on your struct.

SirGuy · Accepted Answer · 2013-09-11 18:24:28Z

If you add/subtract/multiply...etc two numbers together the compiler knows what you mean and everything's fine. If you want to subtract something that's a class, the compiler doesn't inherently know how to do that (for ANY class) but gives you a tool to define how to subtract two classes together and it looks like this (using your Point class):

 Point operator-(Point p1, Point p2) { Point result; result.x = p1.x - p2.x; result.y = p1.y - p2.y; return result; }

Now the compiler knows how to subtract two points, by calling this function.

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