在构造函数中相互引用的两个类

Question

I want to create one 2-dimensional vector datatype and one 3-dimensional vector datatype, but both of them should be compatible to eachother.

Here are the class definitions

class vec2;
class vec3;

class vec2
{
public:
    FLOAT x, y;

    vec2(FLOAT X = 0, FLOAT Y = 0)
    {
        x = X;
        y = Y;
    }

    // constructor declaration (necessary because of "use of undefined type")
    vec2(vec3 V);
};

class vec3
{
public:
    FLOAT x, y, z;

    vec3(FLOAT X = 0, FLOAT Y = 0, FLOAT Z = 0)
    {
        x = X;
        y = Y;
        z = Z;
    }

    vec3(vec2 V)
    {
        x = V.x;
        y = V.y;
        z = 0;
    }
};

vec2::vec2(vec3 V)
{
    x = V.x;
    y = V.y;
}

I want to be able to call a function like this:

// prototypes
int func_Vec2(vec2 Position);
int func_Vec3(vec3 Space);

// create variables
vec2 myPosition(100, 100);// creates a vector "myPosition" with x = 100, y = 100
vec3 mySpace(100, 200, 300);// creates a vector "mySpace" with x = 100, y = 200, z = 300

// call functions with compatible datatypes
func_Vec2(vec2(mySpace));// mySpace is vec3 but the constructor of vec2 allows a vec3 as parameter
func_Vec3(vec3(myPosition));// myPosition is vec2 but the constructor of vec3 allows vec2 as parameter

However, no matter how much I experiment with the class definition / constructors, every try brings another error and I'm running out of ideas.

Because I added

class vec2;
class vec3;

to the top, the following linker error appears: 'LNK2005 "public: __cdecl vec2::vec2(class vec3)" already defined in main.obj'

But if I remove the class prototypes, this error occurs: 'C2061 syntax error vec3' (Because of constructor declaration "vec2(vec3 V);" in class vec2.

Note that I also added

vec2::vec2(vec3 V)
{
    x = V.x;
    y = V.y;
}

outside of the class because vec3 is not yet defined and would throw: "use of undefined type"

Answer 1

Since vec2::vec2(vec3 V) is implemented in the.h file, make it inline .

inline vec2::vec2(vec3 V)
{
    x = V.x;
    y = V.y;
}

Otherwise, every.cpp file that includes the.h file has a non-inline implementation of the constructor. That causes the problem reported by the linker.

The other option will be to move the definition to a.cpp file (and not use inline ).