trouble with accessing a class from a different class - c++

Question

just started to learn c++.I'm trying new things in c++ on thing i wanted to try is to access a class from another class and change its instances and print its instance on screen.
I would like to know 2 things 1)whats wrong with my code 2)where should i declare class declarations (in main file or class definition file?)
here is the error log -
'object::carrier' uses undefined class 'sub'
'cout': is not a member of 'std'
'cout': undeclared identifier
this is what i came up with-
source.h
#include <iostream>
#include <vector>
#include "stuff.h"
int main()
{
object spoon(3);
spoon.get();
}
stuff.cpp
#pragma once
#include <vector>
class object;
class sub;
class object
{
private:
std::vector <sub> thing;
public:
object(int n);
void get() const;
};
class sub
{
private:
int num;
public:
void set_num(int n);
};
stuff.cpp
#include <vector>
#include "stuff.h"
// methods for object
object::object(int n)
{
sub carrier;
carrier.set_num(n);
}
void object::get() const
{
std::cout << carrier.num;
}
// methods for sub
void sub::set_num(int temp_num)
{
num = temp_num;
}
thanks

Answer 1

In your object class, specifically object::get definitions, you use the variable carrier without it being in scope.
When you declare the variable sub carrier in your constructor, it is only accessible in the same scope, that is, inside the constructor. Once your program leaves the scope, the variable carrier is deallocated (deleted).
You must add the variable sub carrier as a member to your class like so:
class object
{
private:
sub carrier
// other stuff
}
Edit:
I so you edited your question.
You must either replace cout with std::cout because cout is part of the c++ standard library. Alternatively, a less verbose option would be to add using namespace std; at the top of every .cpp file. This basically tells the compiler that you can use the namespace std without explicitly saying it. But don't do it for .h files. It's not a good idea.