Error: `undefined reference` to function with array and function pointer as parameters

Each time I call the force function I get an undefined reference; collect2: error: ld returned 1 exit status.

Here is the declaration of the force function above the main function in main.cpp:

void force(Particle(&a_p)[SIZE], void(*force_func)(Particle&, Particle&));

Here is the main function in main.cpp:

int main() {

    Particle a_particle[SIZE];
    
    for (int i = 0; i++ < iterations;) {
        force(a_particle, float3::graviton);
        force(a_particle, float3::photon);
    }
    
    return 0;
}

Here is the definition of the force function below the main function in main.cpp:

void force(Particle(&a_p)[SIZE], void (*force_func)(const Particle&, const Particle&)) {
    for (unsigned int i = 0    ; i++ < SIZE - 1;)
    for (unsigned int j = i + 1; j++ < SIZE;    ) {
        force_func(a_p[i], a_p[j]);
    }
}

SIZE is a definition used for the size of the a_particle array and it's uses (Mine is set to 10).

Both float3 and Particle are structs declared in separate header files.

I tried initializing the function pointers separately as variables and passing them onto the function but that also did not work. Could anyone help me find why I still get this error?

If I am missing any required information I am happy to give more.

    // main.cpp

    #include "float3.h" // Include header file where float3 struct is declared
    #include "Particle.h" // Include header file where Particle struct is declared

    #define SIZE 10 // Define the size of the a_particle array

    void force(Particle(&a_p)[SIZE], void(*force_func)(const Particle&, const Particle&));

    int main() {
    Particle a_particle[SIZE];
    
    // Assuming iterations is defined somewhere
    int iterations = 10; // For example

    for (int i = 0; i++ < iterations;) {
        force(a_particle, float3::graviton);
        force(a_particle, float3::photon);
    }
    
    return 0;
}

void force(Particle(&a_p)[SIZE], void (*force_func)(const Particle&, const Particle&)) {
    for (unsigned int i = 0; i < SIZE - 1; ++i)
        for (unsigned int j = i + 1; j < SIZE; ++j) {
            force_func(a_p[i], a_p[j]);
        }
}