內部類其實就是一種在類聲明裡面定義的一種區域性資料型別。

---- 內部類的宣告有public和private之分

如果宣告為public，那麼外面也可以用它來定義變數，比如Outer::Inner var

如果宣告為private，那麼外面不能用來定義變數，那麼Outer::Inner var將會導致編譯錯誤。

---- 內部類宣告完之後就可以用來定義變數

這就和別的資料型別定義變數一樣了，訪問規則也一樣。無他

---- 內部類和外部類的互相訪問

不能訪問, 完全依賴於成員變數的定義屬性。

---- For example

     1  #include <iostream>
     2  using namespace std;
     3 
     4  class A
     5  {
     6  public:
     7          class B1
     8          {
     9                  public: int a;
    10                  private: int b;
    11                  public: void foo(A &p) {
    12                                  cout << p.i1 << endl;      // OK, because i1 is public in class A
    13                                  cout << p.i2 << endl;      // Fail, because i2 is private in class A
    14                                  }
    15          };
    16 
    17  private:
    18          class B2
    19          {
    20                  public: int a;
    21                  private: int b;
    22                  public: void foo(A &p) {
    23                                  cout << p.i1 << endl;      // OK, because i1 is public in class A
    24                                  cout << p.i2 << endl;      // Fail, because i2 is private in class A
    25                                  }
    26          };
    27 
    28  public:
    29          B1 b11;
    30          B2 b12;
    31          int i1;
    32  private:
    33          B1 b21;
    34          B2 b22;
    35          int i2;
    36  public:
    37                  void f(B1& p) {
    38                  cout << p.a << endl;      // OK, because a is public in class B1
    39                  cout << p.b << endl;      // Fail, because b is private in class B1
    40                  }
    41                  void f(B2& p) {
    42                  cout << p.a << endl;      // OK, because a is public in class B2
    43                  cout << p.b << endl;      // Fail, because b is private in class B2
    44                  }
    45  };
    46 
    47  int main(int argc, char *argv[])
    48  {
    49      A a ;
    50          A::B1 ab1;                              // OK, because B1 is declared as public inner class.
    51          A::B2 ab2;                              // Fail, because B2 is declared as private inner class
    52      return 0;
    53  }