How to call class object in main that is declared in header?

It looks like you badly misuse header files. Also, you mess up unrelated concepts: separate compilation, access of instances and memory allocation.

It is very bad idea to put any objects in a header file. Put only types and method declarations (and some inline definitions, see below). Just thing of how include works and you will understand why.

This is a usual pattern:

MyClass.h:

class MyClass {
public:
	MyClass(int value) : m_Value(value) { }
	int Value() const { return m_Value; }
	int SomeSeriousMethod();
private:
	int m_Value;
};

MyClass.cpp:

#pragma once
#include <stdafx.h> //only if you use pre-compiled headers
#include "MyClass.h"

int MyClass::SomeSeriousMethod() { /*...*/ }

Usage:

#include "MyClass.h"

//..

void SomeFunction() {
	MyClass instanceOnStack(1);
	int value1 = instanceOnStack.SomeSeriousMethod();
	MyClass * instanceInHeap = new MyClass(2);
	int value2 = instanceInHeap->SomeSeriousMethod();
        //...
        //you can access existing stack instance by pointer as well:
	MyClass * someOtherInstance = &instanceOnStack;
	value2 = someOtherInstance->SomeSeriousMethod();
        //...
	//important; better do it under try-finally; this is a "finally" part:
        delete instanceInHeap;
}

—SA