有时候可能需要早定义结构使用struct,可以重复struct的名字，像struct名和typedef一样

//: C03:SelfReferential.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
// Allowing a struct to refer to itself

typedef struct SelfReferential {
  int i;
  SelfReferential* sr; // Head spinning yet?
} SelfReferential;

int main() {
  SelfReferential sr1, sr2;
  sr1.sr = &sr2;
  sr2.sr = &sr1;
  sr1.i = 47;
  sr2.i = 1024;
} ///:~