forward declaration problems - c++

Question

I think I am having a problem with forward declarations. I think one is necessary, but I'm not sure.
Basically I have a main.cpp:
//main.cpp
#include <iostream>
#include "CalculateForces.h"
#include "ParticleBox.h"
int main(void)
{
//g++ main.cpp ParticleBox.cpp -lgsl -lgslcblas -lm -std=c++0x
CalculateForces* calculate_forces= new CalculateForces();
ParticleBox* particles_box = new ParticleBox(2000,100,100,100);
delete calculate_forces;
delete particles_box;
return 0;
}
CalculateForces.h looks like this:
//CalculateForces.h
//We update the forces on each particle
class ParticleBox;
class CalculateForces
{
public:
CalculateForces(void);
~CalculateForces(void);
int UpdateForces(ParticleBox* particlebox);
int DiscretizeSpace(float cutoff_distance);
int LJForce(int local_index, int remote_index, ParticleBox* particlebox);
};
And finally the ParticleBox.h File looks like this:
//ParticleBox.h
//This is the definition of the particlebox. We manage all the particles in this
//file
//This should be changed to a template so that we can run float and double calcs properly :D
struct Particle;
class ParticleBox
{
public:
ParticleBox(int Num_Particles, float Box_length_x_, float Box_length_y_, float Box_length_z_);
~ParticleBox(void);
int set_num_particles(int Num_Particles);
int InitialiseUniverse(int temp,float mass);
float Boltzmann(float temperature);
int GenerateRandomUniquePositions(int number, float max, float min, float* rand_dim_positions);
private:
//Array to hold particles. Each particle has its own struct
Particle** particle_list_;
int num_particles_;
float box_length_x_;
float box_length_y_;
float box_length_z_;
float* rand_x_positions_;
float* rand_y_positions_;
float* rand_z_positions_;
float cutoff_distance_;
float sigma_;
float epsilon_;
};
int CalculateForces::DiscretizeSpace(float cutoff_distance, ParticleBox* particlebox)
{
......
return 0;
}
I use a forward declaration in ParticleBox.h of the Particle Struct and I can add a pointer of type Particle* to the class. This works fine.
The forward in CalculateForces.h of Class ParticleBox causes loads of compiler errors (too many to post but they start in an identical way to the below). Omitting it produces only a few errors:
In file included from main.cpp:3:0:
CalculateForces.h:9:20: error: ‘ParticleBox’ has not been declared
CalculateForces.h:11:50: error: ‘ParticleBox’ has not been declared
In file included from CalculateForces.cpp:3:0:
CalculateForces.h:9:20: error: ‘ParticleBox’ has not been declared
CalculateForces.h:11:50: error: ‘ParticleBox’ has not been declared
CalculateForces.cpp:12:35: error: ‘int CalculateForces::UpdateForces’ is not a static member of ‘class CalculateForces’
CalculateForces.cpp:12:35: error: ‘ParticleBox’ was not declared in this scope
CalculateForces.cpp:12:48: error: ‘particlebox’ was not declared in this scope
CalculateForces.cpp:13:1: error: expected ‘,’ or ‘;’ before ‘{’ token
I thought i would need the forward declaration as I try to use that type as an argument? What am i doing wrong?
Thanks and sorry for the long post

Answer 1

Your post is quite confusing because you posted the error that show up when you omit the forward declaration and you obviously have some additional errors in your code that mix with the error you asked about.
I assume that with the forward declaration, the errors change as they appear mostly in the implementation files, right? In this case the problem might be that the forward declaration is enough as long as you declare a pointer to the type, but it is not enough when you start using the pointer (dereferencing it).
If that is the case, the problem is most likely that you forgot to #include "ParticleBox.h" in CalculateForces.cpp (or some other implementation files).

Answer 2

As Rob determined the source of the errors was in a bit of code that I did not post. There were a few errors but the biggest was that in CalculateForces.cpp I tried to access int num_particles_; which is of course a private member of ParticleBox.