// .h

class class1;

class class2 {

  class2();

  std::unique_ptr<class1> ptr;
}

// .cpp

#include "class1.h"

class2::class2()
{
}

#include <memory>

class Class2;

class Class1
{
public:
    Class1();
private:
    std::unique_ptr<Class2> obj;
};

#include "Class1.h"

class Class2
{
};


Class1::Class1()
{

}

#include <memory>

class Class2;

class Class1
{
public:
    Class1();
private:
    std::unique_ptr<Class2> ptr;
};

#include "Class1.h"
#include "Class2.h"

Class1::Class1()
{
}

class Class2
{
};

V>// .h

V>class class1;

V>class class2 {

V>  class2();

V>  std::unique_ptr<class1> ptr;
V>}

V>// .cpp

V>#include "class1.h"

V>class2::class2()
V>{
V>}
V>

// .h
class Class2;

class Class1 {
  std::unique_ptr<Class2> ptr_member_{new Class2{}};
  ...
public:
  Class1();
  ~Class1();
  ...
};

// .cpp
class Class2 {...}

Class1::Class1() {}
Class1::~Class1() {}

template <typename T>
class MyPtr {
  T *ptr;

  public: 
  MyPtr() {
    ptr = new T();
  }
};

class Class2;

class Class1 {
  MyPtr<Class2> c2ptr;

  Class1() {}
};

class Class2 { };

A specialization for a function template, a member function template, or of a member function or static data member of a class template may have multiple points of instantiations within a translation unit, and in addition to the points of instantiation described above, for any such specialization that has a point of instantiation within the translation unit, the end of the translation unit is also considered a point of instantiation.