c++ calling a class within a class - c++

Question

So i had the following code which ran perfectly.
Here "rngSource" makes an instance of the class contained in rand.h. rng.rFloat64() calls random numbers between 0 and 1.
main.cpp
#include "rand.h" // rngSource
rngSource rng;
class particle{
public:
double r[nd], v[nd];
particle()
{
for (int i=0; i<nd; ++i)
{
r[i]=L*rng.rFloat64();
v[i]=rng.rFloat64();
}
}
};
But when I tried to separate the class implementation from main.cpp into particle.h and particle.cpp, as indicated below:
particle.h
#ifndef particle_H
#define particle_H
class particle{
public:
double r[2], v[2];
particle();
};
#endif
particle.cpp
#include "rand.h" // rngSource
#include "particle.h"
particle::particle()
{
double ran = (double) rand()/(double)RAND_MAX;
static const double L=10;
for (int i=0; i<2; ++i)
{
r[i]=L*ran;
v[i]=ran;
}
}
While the new main.cpp looks like this:
#include "rand.h" /* rngSource() */
#include "particle.h" /* particle class */
rngSource rng;
int main(){
rng.rseed(getpid()*time(NULL));
particle p[N];
....
}
But when i try to compile i get the following errors:
particle.cpp: In constructor ‘particle::particle()’:
particle.cpp:20:17: error: ‘rng’ was not declared in this scope
r[i]=L*rng.rFloat64(); //ran;
^
particle.o:(.data+0x0): multiple definition of `rng_cooked'
new.o:(.data+0x0): first defined here
particle.o:(.data+0x1300): multiple definition of `kn'
new.o:(.data+0x1300): first defined here
particle.o:(.data+0x1500): multiple definition of `wn'
new.o:(.data+0x1500): first defined here
particle.o:(.data+0x1700): multiple definition of `fn'
new.o:(.data+0x1700): first defined here
new.cpp:(.text+0x1fb1): undefined reference to `rngSource::rseed(long long)'
particle.o: In function `particle::particle()':
particle.cpp:(.text+0x1fa2): undefined reference to `rngSource::rseed(long long)'
particle.cpp:(.text+0x1fb5): undefined reference to `rngSource::rFloat64()'
particle.cpp:(.text+0x1fda): undefined reference to `rngSource::rFloat64()'
collect2: error: ld returned 1 exit status
Would anyone here happen to know how to fix this?
thank you.

Answer 1

Three problems.
Problem 1:
particle.cpp: In constructor ‘particle::particle()’:
particle.cpp:20:17: error: ‘rng’ was not declared in this scope
r[i]=L*rng.rFloat64(); //ran;
The ultra cheesy answer is to add extern rngSource rng; to particle.cpp above the particle constructor. extern means this variable exists, but its storage is allocated elsewhere. The linker will track it down for you. This is quick, dirty, and will get you back on the road with minimal code changes.
Better solutions are to add extern rngSource rng; to rand.h and then define rngSource rng; in rand.cpp for all users or placing extern rngSource rng; in a brand new main.h and include main.h in particle and other modules called by main.
The best solution is probably to create a bool init(rngSource & rng) function in particle and do grunt work the constructor's currently doing. That way if rng doesn't exist, the compiler catches it when you try to call init.
particle p[N];
for (size_t index; index < N; index++)
{
p[index].init(rng);
}
Problem 2
particle.o:(.data+0x0): multiple definition of `rng_cooked'
new.o:(.data+0x0): first defined here
particle.o:(.data+0x1300): multiple definition of `kn'
new.o:(.data+0x1300): first defined here
particle.o:(.data+0x1500): multiple definition of `wn'
new.o:(.data+0x1500): first defined here
particle.o:(.data+0x1700): multiple definition of `fn'
new.o:(.data+0x1700): first defined here
rng_cooked, kn, wn, and fn are almost certainly being defined in rand.h. This means everyone who includes rand.h tries to create their own version of those variables resulting in collisions when the linker tries to line up names with storage locations.
Solution is similar to the first problem: Add extern to the variable definitions in the rand.h and define them in rand.cpp.
Problem 3
new.cpp:(.text+0x1fb1): undefined reference to `rngSource::rseed(long long)'
particle.o: In function `particle::particle()':
particle.cpp:(.text+0x1fa2): undefined reference to `rngSource::rseed(long long)'
particle.cpp:(.text+0x1fb5): undefined reference to `rngSource::rFloat64()'
particle.cpp:(.text+0x1fda): undefined reference to `rngSource::rFloat64()'
No idea. Are you certain you are passing rand.cpp to gcc?