putting some class methods in different .cc file - c++

Question

I have a bit of code of the following format contained in a single .h and .cc file:
myClass.h:
#ifndef MYCLASS_H
#define MYCLASS_H
class myClass
{
public:
myClass(); // constructor
~myClass(); // destructor
void classMethod1 ();
void classMethod2 ();
int memberVarable1;
int memberVariable2;
};
#endif
and myClass.cc:
#include "myClass.h"
myClass::myClass(){
// stuff
}
myClass::~myClass(){
// stuff
}
void myClass::classMethod1 (){
// stuff
}
void myClass::classMethod2 (){
// stuff
}
All of this is working fine. However my project is getting quite large and I'm about to add a set of new functionality. Instead of clogging up myClass.h and myClass.cc I want to put some new methods in another .cc file. I don't seem to be able to get this to work though.
myClass.h:
#ifndef MYCLASS_H
#define MYCLASS_H
#include "secondFile.h"
class myClass
{
public:
myClass(); // constructor
~myClass(); // destructor
void classMethod1 ();
void classMethod2 ();
int memberVarable1;
int memberVariable2;
};
#endif
and myClass.cc:
#include "myClass.h"
#include "secondFile.h"
myClass::myClass(){
// stuff
}
myClass::~myClass(){
// stuff
}
void myClass::classMethod1 (){
// stuff
}
void myClass::classMethod2 (){
// stuff
}
secondFile.h:
#ifndef SECONDFILE_H
#define SECONDFILE_H
void someNewMethod();
#endif
secondFile.cc
#include "secondFile.h"
void someNewMethod(){
// can't see classMethod1()
}

Answer 1

In every source file, you need to include every header file that declares functions, etc. you want to use.
So in your case, it seems like you want secondFile.cc to contain
#include "myClass.h"
#include "secondFile.h"
void someNewMethod(){
// can't see classMethod1()
}
Btw, what you are doing is quite common to do in practice. Sometimes, I go even further than what you suggest, and implement the various methods of a single class in multiple source files. For large, complicated classes, this helps speed up the development cycle because I only have to recompile a fraction of the class implementation if I only made a small change. Example:
myclass.h
#pragma once
class MyClass
{
...
void complicatedMethod0();
void complicatedMethod1();
...
};
myclass_impl0.cpp
#include "myclass.h"
void MyClass::complicatedMethod0()
{
...
}
myclass_impl1.cpp
#include "myclass.h"
void MyCLass::complicatedMethod1()
{
...
}

Answer 2

If you are intending to add methods to myClass, you can't do that - the class methods have to be contained in one definition.
You can extend myClass, however, by inheriting from it:
secondFile.h:
#ifndef SECONDFILE_H
#define SECONDFILE_H
#include "myClass.h"
class mySecondClass : public myClass
{
public:
void someNewMethod();
}
#endif
secondFile.cc
#include "secondFile.h"
void mySecondClass::someNewMethod(){
this.classMethod1();
}

Answer 3

#include "secondFile.h"
#include "myClass.h"
//if you want the class methods,
//you need to tell the compiler where to look
void someNewMethod(){
// can't see classMethod1()
}
it seems you've forgotten to include "myClass.h".