How to use instance of one class in a method of another class?

I defined two classes class A and class B . They are completely independent.

Now I create c as instance of class B and d as instance of class A . Now I want to define the body of functionOfA that will perform operations on c :

class A {
public:
    functionOfA();
}

class B {
    ...
}

A::functionOFA()
{
    c.functionOfB();
}

main()
{
    A d;
    B c;
    d.functionOfA();
}

but the compiler gives me the following error: c is not declared in this scope

---

A::functionOfA()定义需要知道什么是c实例，你可以传入B实例：

class B;

class A
{
public:
    functionOfA(const B& b) const;
};

A::functionOFA(const B& b) const
{
    b.functionOfB();
}

A a;
B b;
a.functionOfA(b); // pass in B instance

---

In this code (your main ):

{
    A a;
    B b;
    a.functionOfA();
}

b is a local variable usable only within this scope. b represents an object with automatic storage duration that exists until the execution goes out of the scope (in this case: out of your main ).

When you call the method functionOfA , although the object b still "lives", the functionOfA has no means of accessing this object ~> this method needs a reference to this object (or its copy) to use it:

class A {
public:
    void functionOfA(B& b) {
        /* b is now accessible here, this method can also change the object b */
    }

called in this manner:

A a;
B b;
a.functionOfA(b);

I recommend you to also have a look at these questions:
How to pass objects to functions in C++?
Is it better in C++ to pass by value or pass by constant reference?
Are there benefits of passing by pointer over passing by reference in C++?

and some good book might be very helpful here too:
The Definitive C++ Book Guide and List

---

A::functionOFA()
{
    c.functionOfB();  //You cannot do this unless c is global
}

However in your case its in main

Use:

 B c;
void A::functionOfA()
{
    c.functionOfB();
}

OR

pass Object of B as an argument functionOfA(const B& c)

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