Undefined reference to a class in C++ - c++

Question

I have a code::blocks project, called Test.cbp. It has three files, main.cpp in the Test directory, and jAlg.cpp and jAlg.h, both in the jAlg directory.
main.cpp
#include <iostream>
#include "jAlg.h"
int main(){
jVector bob;
jMatrix ann;
bob.display();
return 0;
}
jAlg.cpp (summary)
class jMatrix{
public:
//The four entries of a 2x2 square matrix:
//[a b]
//[c d]
double a, b, c, d;
//Constructor
jMatrix(double a_val = 1, double b_val = 0,
double c_val = 0, double d_val = 1){
a = a_val;
b = b_val;
c = c_val;
d = d_val;
}
//Several other functions and operator overloads
};
class jVector{
public:
//The two entries of a 2D vector
//[x]
//[y]
double x, y;
//Constructor
jVector(double x_val = 0,
double y_val = 0){
x = x_val;
y = y_val;
}
//Several other functions and operator overloads
};
//Several other functions
//Matrix-vector multiplication
jVector operator* (jMatrix A, jVector v){
jVector result;
result.x = A.a*v.x + A.b*v.y;
result.y = A.c*v.x + A.d*v.y;
return result;
}
//More functions
jAlg.h (summary)
#ifndef JALG
#define JALG
double sqr(double x);
class jMatrix{
public:
double a, b, c, d;
jMatrix(double a_val = 1, double b_val = 0,
double c_val = 0, double d_val = 1);
//more functions
};
class jVector{
public:
double x, y;
jVector(double x_val = 0, double y_val = 0);
//more functions
};
jVector operator* (jMatrix A, jVector v);
#endif
When I build and run the project, it behaves strangely. An error is raised saying undefined reference to 'jMatrix::jMatrix(double,double,double,double)'. However, there is no error related to the jVector::jVector constructor.
As a test I went to the jAlg files and commented out the jVector operator* (jMatrix A, jVector v) function and run the project again. This time, an error is raised for both jMatrix::jMatrix and jVector::jVector. For some reason, the presence of that function makes it okay to define vectors, but not matrices.
So here's my question: why does the presence of the function jVector operator* (jMatrix A, jVector v) make it okay to define jVectors? What can I do to make it possible to define jMatrix objects?

Answer 1

In your jAlg.cpp file, you should only define your functions, don't declare everything again
jMatrix::jMatrix(double a_val, double b_val, double c_val, double d_val)
: a{a_val}, b{b_val}, c{c_val}, d{d_val}
{ }
// ... define other jMatrix functions similarly
jVector::jVector(double x_val, double y_val)
: x{x_val}, y{y_val}
{ }
// ... define other jVector functions
//Several other functions
jVector operator* (jMatrix A, jVector v)
{
jVector result;
result.x = A.a*v.x + A.b*v.y;
result.y = A.c*v.x + A.d*v.y;
return result;
}

Answer 2

In your jAlg.cpp file you shouldn't redeclare your class, but just put definitions for the functions declared from jAlg.h (do so for the other class member functions analogous):
jMatrix::jMatrix(double a_val, double b_val,
// ^^^^^^^^^ Prefix class scope
double c_val, double d_val)
: a(a_val), b(b_val), c(c_val), d(d_val) {}
Also use the member initializer list (as shown above).
The definition of the free function jVector operator* (jMatrix A, jVector v); is OK.