Error in linking to friend functions

Question

I have a class 'Vector3' which is compiled successfully. It contains both non-friend and friend functions, for example, to overload * and << operators when Vector3 is the second operand. The problem is I can't link to any of the friend functions, be it operator overloaded or not. So I can confirm that the error is not specific to operator overloading. The g++ command used for linking is as follows (please also see Makefile at the end),

g++ -Wall -W -I./ -g -o main.out main.o Vector3.o

which gave the following errors,

main.cpp:7: undefined reference to `operator*(double, Vector3 const&)'
main.cpp:9: undefined reference to `mag(Vector3 const&)'
main.cpp:10: undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, Vector3 const&)'

Below is the relevant code in my source files. I follow the practice of making separate .hpp and .cpp for every class.

/* file Vector3.hpp */
struct Vector3 {
    ...
    Vector3 operator*(const double k) const;
    friend Vector3 operator*(const double k, const Vector3 &vec);
    double magnitude() const;
    friend double mag(const Vector3 &vec);
    friend std::ostream& operator<<(std::ostream& output, const Vector3 &vec);
    ...
}

/* file Vector3.cpp */
Vector3 operator*(const double k, const Vector3 &vec) {
    ...
}

inline double mag(const Vector3 &vec) {
    ...
}

std::ostream& operator<<(std::ostream& output, const Vector3 &vec) {
    ...
}

/* file main.cpp */
#include "Vector3.hpp"
int main() {
    Vector3 M(1, 1, 1);
    M = M * 2.0;              // own operator* links successfully
    M = 10.0 * M;             // friend operator* doesn't link
    double m = M.magnitude(); // own function magnitude() links successfully
    double n = mag(M);        // friend function mag() doesn't link
    std::cout << M;           // friend operator<< doesn't link
}

Finally, this is my Makefile.

CXX         = g++
CXXFLAGS    = -Wall -W $(INCPATH) -g
INCPATH     = -I./
OBJS        = main.o Vector3.o

main.out: $(OBJS)
 $(CXX) $(CXXFLAGS) -o $@ $(OBJS) $(LIBPATH)

main.o: main.cpp
Vector3.o: Vector3.cpp

clean:
 rm -f $(OBJS) main.out

The strangest thing is that if I include the Vector3.cpp file as well in main.cpp and then remove Vector3.o from OBJS in Makefile, the program links successfully. I cannot make sense of this. Please help me!!

Answer 1

The definition of friend operator* uses fp_type while the friend declaration uses double as the first parameter. This will only work as intended if fp_type is a typedef-name for double. Are you sure fp_type actually stands for double? I can't see it from the code you posted.

The problem with mag is rather obvious: you defined it as inline in .cpp file. Inline function definitions have to be visible everywhere they are used, meaning that normally they should be placed in the header file.