bitwise const

最新推荐文章于 2023-07-04 11:22:17 发布

原创最新推荐文章于 2023-07-04 11:22:17 发布 · 1.1k 阅读

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本文探讨了C++中常量对象(const object)的概念及其如何与mutable成员变量配合使用来实现某些特殊功能，例如在不改变常量对象本身的情况下更新其内部状态。通过具体的代码示例展示了如何正确地声明和使用mutable关键字，并解释了为什么这样做是安全且必要的。

class CTextBlock {

public:

  ...



  char& operator[](std::size_t position) const   // inappropriate (but bitwise

  { return pText[position]; }                    // const) declaration of

                                                 // operator[]

private:

  char *pText;

};

const CTextBlock cctb("Hello");        // declare constant object



char *pc = &cctb[0];                   // call the const operator[] to get a

                                       // pointer to cctb's data



*pc = 'J';                              // cctb now has the value "Jello"


class CTextBlock {

public:



  ...



  std::size_t length() const;



private:

  char *pText;

  std::size_t textLength;            // last calculated length of textblock

  bool lengthIsValid;                // whether length is currently valid

};



std::size_t CTextBlock::length() const

{

  if (!lengthIsValid) {

    textLength = std::strlen(pText);  // error! can't assign to textLength

    lengthIsValid = true;             // and lengthIsValid in a const

  }                                   // member function



  return textLength;

}

class CTextBlock {

public:



  ...



  std::size_t length() const;



private:

  char *pText;



  mutable std::size_t textLength;         // these data members may

  mutable bool lengthIsValid;             // always be modified, even in

};                                        // const member functions



std::size_t CTextBlock::length() const

{

  if (!lengthIsValid) {

    textLength = std::strlen(pText);      // now fine

    lengthIsValid = true;                 // also fine

  }



  return textLength;

}