讀書心得 - C++ Primer (5th Edition) - Chapter 4 (2) - Type Conversions

Type Conversions

• Implicit Conversions

  型別轉換，大部分轉換都是看是否能轉換成 int，例如 char, bool, 和 short。

      bool      flag;   char           cval;
      short     sval;   unsigned short usval;
      int       ival;   unsigned int   uival;
      long      lval;   unsigned long  ulval;
      float     fval;   double         dval;
      3.14159L + 'a'; //  'a' promoted to int, then that int converted to long double
      dval + ival;    //  ival converted to double
      dval + fval;    //  fval converted to double
      ival = dval;    //  dval converted (by truncation) to int
      flag = dval;    //  if dval is 0, then flag is false, otherwise true
      cval + fval;    //  cval promoted to int, then that int converted to float
      sval + cval;    //  sval and cval promoted to int
      cval + lval;    //  cval converted to long
      ival + ulval;   //  ival converted to unsigned long
      usval + ival;   //  promotion depends on the size of unsigned short and int
      uival + lval;   //  conversion depends on the size of unsigned int and long
  

• Explicit Conversions

  用於我們想明確地強制將 object 轉換為其他類型。cast-name\(expression)。(const相關文章1, const相關文章2)

      // static_cast 最常用的 cast-name
      // cast used to force floating-point division
      double slope = static_cast<double>(j) / i;
  
      // const_cast 用來改變 low-level const (point to const)
      const char *pc;
      char *p = const_cast<char*>(pc); // ok: but writing through p is undefinedc
  
      // reinterpret_cast 對其 operand 進行 low-level bit 的重新解釋
      // 要很理解其型別的 bit 組成才會使用
      int *ip;
      char *pc = reinterpret_cast<char*>(ip);
  

cast 基本上就是擾亂型別，建議能不使用就不使用。