现在有两个模板类（头文件A.h为1~14行，头文件B.h为15~27行）：

  1: ///////////////////////////////////////////
  2: // file A.h
  3: //
  4: #include "B.h"
  5: 
  6: template <class T>
  7: class A
  8: {
  9: public:
 10:     T a;
 11:     B b_ptr;
 12:     A(): a(0), b_ptr(NULL) {}
 13: };
 14: 
 15: ///////////////////////////////////////////
 16: //file B.h
 17: //
 18: #include "A.h"
 19: 
 20: template <class T>
 21: class B
 22: {
 23: public:
 24:     T b;
 25:     A a_ptr;
 26:     B(): b(0), b_ptr(NULL) {}
 27: };

解决办法：

在A.h中对类B进行前向声明，在B.h中对类A进行前向声明，如下代码所示（代码7~8行和25~26行）：

  1: ///////////////////////////////////////////
  2: // file A.h
  3: //
  4: #include "B.h"
  5: 
  6: // forward statement
  7: template <class T>
  8: class B;
  9: 
 10: template <class T>
 11: class A
 12: {
 13: public:
 14:     T a;
 15:     B b_ptr;
 16:     A(): a(0), b_ptr(NULL) {}
 17: };
 18: 
 19: ///////////////////////////////////////////
 20: //file B.h
 21: //
 22: #include "A.h"
 23: 
 24: // forward statement
 25: template <class T>
 26: class A;
 27: 
 28: template <class T>
 29: class B
 30: {
 31: public:
 32:     T b;
 33:     A a_ptr;
 34:     B(): b(0), b_ptr(NULL) {}
 35: };

关键词：

前向声明， 模板类