c++ access main objects from other file - c++

Question

I wonder how it is possible to access an object that was created in the main function from another class.
main.cpp
#include"ClassA.h";
#include"ClassB.h";
int main()
{
ClassA objectA;
return 0;
}
ClassA.h
#pragma once
class ClassA
{
public:
ClassA():_privateVar(100)
{};
~ClassA()
{};
//Getters
float getPrivateVarA(){ return _privateVarA; };
//Setters
void setPrivateVarA(float privateVarA){ _privateVarA = privateVarA; };
private:
//Just a value
float _privateVarA;
};
ClassB.h
#pragma once
class ClassB
{
public:
ClassB():_privateVarB(50)
{ };
~ClassB()
{ };
//This is what i´m trying to achieve: Acces objectA
// like this: objectA.getPrivateVarA(); or objectA.setPrivateVarA;
int getPrivateVarB(){return _privateVarB;};
private:
int _privateVarB;
};
I've been all week searching for an answer to this and found nothing...
If anyone knows of some books or have any information on how I can get there would be grateful.
Thank you.

Answer 1

You placed your question in the middle of the ClassB declaration. You can't execute code there. Whatever you do must be done from within a function.
A function in ClassB can be defined so that it accepts a reference or pointer to a ClassA. Then that reference can be used to call getPrivateVarA().
In other words, if class B needs to access a class A then it is up to your code to initialize B with the required reference. This can be done when creating the B object, or when calling a method of the B object.

Answer 2

objectA has function-local scope within main(). By definition, objects can be directly accessed only within their visible scope. That's what C++ is all about.
Of course, if you pass a reference or a pointer to this object, to some other function, that other function can access the instantiated object indirectly, via the pointer or the reference.

Answer 3

I can't say with 100% certainty that you can't, but you definitely should not be doing that. If you want to use an object created in a different scope then you need to pass it into the scope you want to access it from either by value, reference or via a pointer.
First Class A:
class A {
public:
ClassA() : _privateVar(100) {}
~ClassA() {}
float getPrivateVarA() { return _privateVar; }
void setPrivateVarA(float val) { _privateVar = val; }
private:
float _privateVar;
};
First Class B:
class B {
public:
ClassB() : _privateVar(50) {}
~ClassB() {}
// by copy
float getPrivateVarB_byCopy(ClassA a) {
return _privateVar + a.getPrivateVarA();
}
// by reference
float getPrivateVarB_byRef(ClassA &a) {
return _privateVar + a.getPrivateVarA();
}
// by pointer
float getPrivateVarB_byPointer(ClassA *a) {
return _privateVar + a->getPrivateVarA();
}
float setPrivateVarB(float val) { _privateVar = val; }
private:
float _privateVar;
};
Now for main.
int main(void) {
ClassB b;
ClassA a; // for copy and ref
ClassA *a2 = new ClassA(); // for pointer
b.getPrivateVarB_byCopy(a); // => 150
b.getPrivateVarB_byRef(a); // => 150
b.getPrivateVarB_byPointer(a2); // => 150
delete a2; // clean up pointer
return 0;
}
Although your example this type of access is really not a good idea, not sure why you'd want to go about doing things this way.