将const转换为非const或将const转换为const [英] `const` to non-`const` or non-`const` to `const`

问题描述

当同时存在 const 和非 const 吸气剂时，以下代码用于减少代码重复.它从非 const 创建 const 版本.我对此进行了搜索，很多人说我应该改为从 const 创建非 const 版本.

The code below is used for reducing code duplication when there are both const and non-const getters. It creates the const version from non-const. I searched about this a bit and a lot of people are saying that I should instead create the non-const version from const.

我的想法是，如果我从 const 版本创建非 const 版本，与下面的代码相反，则取消引用返回的指针或最初是 const 的引用.我真的不确定这一点，所以我要澄清一下，什么是正确的方向"?

My thought is that if I create the non-const version from the const version, opposite from the code below, it may be unsafe to dereference the returned pointer or reference which was originally const. I'm really not sure about this, so I'm asking for clarification, and what is the correct 'direction'?

template<typename T>
struct Constifier
{
  typedef T Type;
};

template<typename T>
struct Constifier<T &>
{
  typedef const T &Type;
};

template<typename T>
struct Constifier<T &&>
{
  typedef const T &&Type;
};

template<typename T>
struct Constifier<T *>
{
  typedef const T *Type;
};

template<typename F>
struct ReturnType;

template<typename R, typename ...Ts>
struct ReturnType<R (*)(Ts ...ts)>
{
  typedef R Type;
};

template<typename R, typename T, typename ...Ts>
struct ReturnType<R (T::*)(Ts ...ts)>
{
  typedef R Type;
};

template<typename T, typename F, typename ...Ts>
auto callConstVersion(const T *t, F f, Ts ...ts)
{
  return const_cast<typename Constifier<typename ReturnType<F>::Type>::Type>((const_cast<T *>(t)->*f)(ts...));
}

struct A
{
  A *p;
  A *get() {return p;}
  const A *get() const {return callConstVersion(this, static_cast<A *(A::*)()>(&A::get));}
};

推荐答案

您应始终从 const 创建一个 non-const 版本，而不要相反.解释很简单，请这样想:

You should always create non-const version from const one and not in the opposit way. The explanation is simple, think of it this way:

您只能在 non-const 对象上调用 non-const getter.如果对象是 non-const ，则可以在 non-const 方法内安全地 const_cast 您的 const 吸气剂.为什么?因为您只能在 non-const 对象上调用此方法.

You can call non-const getter only on the non-const object. If the object is non-const, you can safely const_cast your const getter inside the non-const method. Why? Because you can only call this method on non-const object.

进行相反的操作并不真正安全，因为在 const 中调用 non-const 方法时，您不能保证保持不变.

Doing the opposit is not really safe, because when calling non-const method in the const one you cannot assure, that constness is preserved.

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