The class template references the problem (error: The type specifier is missing-assumed to be int). Note: C + + does not support the default int)

Problem Description:

There are now two template classes (header file A.h is 1~14 line, header file B.h is 15~27 line):

  ///////////////////////////////////////////
  File A.h
  //
  4: #include "B.h"
  
  Template <class t>
  class A
  8: {
  Public:
Ten:     T A;
One:     B b_ptr;
:     A (): A (0), b_ptr (NULL) {}
13:};
///////////////////////////////////////////
File B.h
//
: #include "A.h"
Template <class t>
class B
22: {
Public:
:     T B;
:     A a_ptr;
+:     B (): B (0), b_ptr (NULL) {}
27:};

The compilation here will report the following error:

Error C4430: Missing type descriptor-assumed to be int. Note: C + + does not support the default int

Error C2143: syntax error: Missing "," (in front of "<")

Workaround:

Forward Declaration of Class B in A.h, a forward declaration of Class A in B.h, as shown in the following code (code 7~8 lines and 25~26 lines):

  ///////////////////////////////////////////
  File A.h
  //
  4: #include "B.h"
  
  Forward statement
  Template <class t>
  class B
  
Template <class t>
class A
12: {
Public:
:     T A;
:     B b_ptr;
:     A (): A (0), b_ptr (NULL) {}
17:};
///////////////////////////////////////////
File B.h
//
: #include "A.h"
Forward statement
Template <class t>
class A
Template <class t>
class B
30: {
Public:
C:     T B;
:     A a_ptr;
:     B (): B (0), b_ptr (NULL) {}
35:};

Keywords:

Forward declaration, template class

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The class template references the problem (error: The type specifier is missing-assumed to be int). Note: C + + does not support the default int)