在类外定义重载的下游运算符

Question

Here is some simple code that I wrote. It simply copies an object and displays it data functions with an overloaded operator.

      //Base
      #include<iostream>
      #include<istream>
      #include<ostream>
      using std::ostream;
      using std::istream;
      using namespace std;

      class Sphere{
      public: 

      Sphere(double Rad = 0.00, double Pi = 3.141592);


      ~Sphere();




    Sphere(const Sphere& cSphere)

    //overloaded output operator
    friend ostream& operator<<(ostream& out, Sphere &fSphere);


    //member function prototypes
    double Volume();
    double SurfaceArea();
    double Circumference();

protected:
    double dRad;
    double dPi;
};


//defining the overloaded ostream operator
ostream& operator<<(ostream& out, Sphere& fSphere){
    out << "Volume: " << fSphere.Volume() << '\n'
        << "Surface Area: " << fSphere.SurfaceArea() << '\n'
        << "Circumference: " << fSphere.Circumference() << endl;
    return out;
    }

The member functions are defined in a .cpp file. The problem is that when I compile this program I am told

 there are multiple definitions of operator<<(ostream& out, Sphere& fSphere)

This is strange because the outstream operator is a non-member function so it should be able to be defined out of the class. Yet the program works well when I define this operator inside the class. Whats going on?

Answer 1

It seems you defined the operator in a header file and include this header in multiple cpp modules. or you include one cpp module with the function definition in other cpp module. Usually the error mesage shows where a function is multiple defined. So reread all lines of the error message

Take into account that it would be better to declare the operator as

ostream& operator<<(ostream& out, const Sphere &fSphere);

Answer 2

Looks like the code you presented if the header file. And it contains the definition of operator<< , so any file including your header has its own copy of this definition, hence "multiple definitions" error. Add the keyword inline to your function, or move the function to .cpp file.