如何删除类似的const和非const成员函数之间的代码复制？

问题描述：

假设我有以下 class X ，我想要返回内部成员的访问权限：

Let's say I have the following class X where I want to return access to an internal member:

class Z
{
    // details
};

class X
{
    std::vector<Z> vecZ;

public:
    Z& Z(size_t index)
    {
        // massive amounts of code for validating index

        Z& ret = vecZ[index];

        // even more code for determining that the Z instance
        // at index is *exactly* the right sort of Z (a process
        // which involves calculating leap years in which
        // religious holidays fall on Tuesdays for
        // the next thousand years or so)

        return ret;
    }
    const Z& Z(size_t index) const
    {
        // identical to non-const X::Z(), except printed in
        // a lighter shade of gray since
        // we're running low on toner by this point
    }
};

两个成员函数 X :: Z（）和 X :: Z（）const 在大括号中有相同的代码。这是重复的代码，可能会对具有复杂逻辑的长函数造成维护问题。

The two member functions X::Z() and X::Z() const have identical code inside the braces. This is duplicate code and can cause maintenance problems for long functions with complex logic.

有没有办法避免这种代码重复？

Is there a way to avoid this code duplication?

答

是的，可以避免代码重复。你需要使用const成员函数具有逻辑并使非const成员函数调用const成员函数并将返回值重新转换为非const引用（如果函数返回指针，则为指针）：

Yes, it is possible to avoid the code duplication. You need to use the const member function to have the logic and have the non-const member function call the const member function and re-cast the return value to a non-const reference (or pointer if the functions returns a pointer):

class X
{
   std::vector<Z> vecZ;

public:
   const Z& Z(size_t index) const
   {
      // same really-really-really long access 
      // and checking code as in OP
      // ...
      return vecZ[index];
   }

   Z& Z(size_t index)
   {
      // One line. One ugly, ugly line - but just one line!
      return const_cast<Z&>( static_cast<const X&>(*this).Z(index) );
   }

 #if 0 // A slightly less-ugly version
   Z& Z(size_t index)
   {
      // Two lines -- one cast. This is slightly less ugly but takes an extra line.
      const X& constMe = *this;
      return const_cast<Z&>( constMe.Z(index) );
   }
 #endif
};

注意： strong>将逻辑放在非const函数中，并使用const函数调用非const函数 - 它可能导致未定义的行为。原因是常量类实例被转换为非常量实例。非const成员函数可能会意外修改类，C ++标准状态将导致未定义的行为。

NOTE: It is important that you do NOT put the logic in the non-const function and have the const-function call the non-const function -- it may result in undefined behavior. The reason is that a constant class instance gets cast as a non-constant instance. The non-const member function may accidentally modify the class, which the C++ standard states will result in undefined behavior.

如何删除类似的const和非const成员函数之间的代码复制？

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