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