如何在左右两侧创建运算符*（double）以进行相乘？

Question

I'm trying to figure out how to write an overloaded operator for my Vector2d class that allows me to multiply by a scalar on both the left and right sides.

class Vector2d
{
    double _x;
    double _y;

public:
    Vector2d(double x = 0, double y = 0) :_x(x), _y(y) {}

    Vector2d operator*(const double s) const
        { return Vector2d(_x * s, _y * s); }

    friend Vector2d operator*(const double s, const Vector2d& v);
};

Vector2d operator*(const double s, const Vector2d& v)
{
    return Vector2d(v._x * s, v._y * s);
}

If I only define the member operator*, my object can be multiplied on the right by a scalar but not the left. If I add the friend function operator*, I get an error when I compile:

Vector2D.h:61: multiple definition of `Gf::operator*(double, Gf::Vector2d const&)'
Vector2D.h:61: first defined here
Vector2D.h:61: multiple definition of `Gf::operator*(double, Gf::Vector2d const&)'

What's the right way to do this?

---

I was putting the operator* function in the header file. Once I moved it to the .cpp, it compiled correctly.

Answer 1

It looks like your file has been included multiple times, most compilers support #pragma once these days. You can also use a header guard (check for a token's definition before defining it along with the rest of your header) :

#ifndef VECTOR_2D
#define VECTOR_2D

class Vector2d
{
    double _x;
    double _y;

public:
    Vector2d(double x = 0, double y = 0) :_x(x), _y(y) {}

    Vector2d operator*(const double s) const
        { return Vector2d(_x * s, _y * s); }

    friend Vector2d operator*(const double s, const Vector2d& v);
};

Vector2d operator*(const double s, const Vector2d& v)
{
    return Vector2d(v._x * s, v._y * s);
}

#endif // VECTOR_2D