The Binding of a Data Member

《深度探索C++对象模型》第三章“Data语意学” 

3.1 The Binding of a Data Member

下面一段代码的输出是什么？

 

[xiaochu.yh@OceanBase cpp]$ cat member_binding.cpp 

#include <iostream>

using namespace std;



typedef int len_t;



class Point

{

  public:

    void mum(len_t val) { _val = val; cout << _val << endl; }

  private:

    typedef float len_t;

    len_t _val;

};



int main()

{

  Point p;

  p.mum(123.3);

  return 0;

}

 

答案是：123

那么下面一段代码呢？

 

[xiaochu.yh@OceanBase cpp]$ cat member_binding.cpp     

#include <iostream>

using namespace std;



typedef int len_t;



class Point2

{

  private:

    typedef float len_t;

  public:

    void mum(len_t val) { _val = val; cout << _val << endl; }

  private:

    len_t _val;

};





int main()

{

  Point2 p2;

  p2.mum(123.3);

  return 0;

}

 

 

答案是：123.3

从上面的案例中可以知道，作为一种防御性编程风格，建议把类体中的typedef放在最前面，如例2中所示。

还看最后一段代码，会是什么输出呢？

 

[xiaochu.yh@OceanBase cpp]$ cat member_binding.cpp 

#include <iostream>

using namespace std;



class Point

{

  public:

    void mum(len_t val) { _val = val; cout << _val << endl; }

  private:

    typedef float len_t;

    len_t _val;

};



int main()

{

  Point p;

  p.mum(123.3);

  return 0;

}

 

答案是：编译失败

member_binding.cpp:7: error: ‘len_t’ has not been declared