How to overload both unary and binary minus operators in C++?

Question

In C++, how are the operator overloading functions distinguished for the unary and binary minus operators?

I am trying to overload both with the following code:

Vector Vector::operator-(){
  return Vector(-x,-y,-z);
}

Vector Vector::operator-(const Vector& v){
  return this* + (-v);
}

But this spews a lot of errors:

vector.cpp: In member function ‘Vector Vector::operator-(const Vector&)’:
vector.cpp:88:20: error: passing ‘const Vector’ as ‘this’ argument of ‘Vector Vector::operator-()’ discards qualifiers [-fpermissive]
   return this* + (-v);
                    ^
vector.cpp:88:16: error: no match for ‘operator+’ (operand type is ‘Vector’)
   return this* + (-v);
                ^
vector.cpp:88:16: note: candidates are:
vector.cpp:70:8: note: Vector Vector::operator+(const Vector&)
 Vector Vector::operator+(const Vector& v){
        ^
...

How do I fix this problem?

Implement as a free function, not a member function. Or (if you insist on implementing as a member function) mark it as a const member function, i.e. Vector Vector::operator-(const Vector&) const; and Vector Vector::operator-() const;` — Andrew
– Andrew, Commented May 27, 2016 at 15:18

songyuanyao · Accepted Answer · 2016-05-27 15:25:44Z

6

1.v is passed by reference to const, it can't be called with non-const member function. Since operator- and operator+ (both unary and binary version) doesn't modify members of class, you should make them const member functions.

Vector Vector::operator-() const {
  return Vector(-x,-y,-z);
}

2.Change

return this* + (-v);

to

return *this + (-v);

edited May 27, 2016 at 15:25

answered May 27, 2016 at 15:20

songyuanyao

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1 Comment

songyuanyao Over a year ago

@brandbest1 It's just a syntax error. this* + (-v); will call unary operator- on v, and call unary operator+ on the returned value, then failed since you only define a binary operator+. *this + (-v) will call unary operator- on v, and *this will dereference this and then call binary operator+ with them.

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