c++

const -> pointer

school

Const was used with Pointer.

  • The first case:

    The address which Pointer contained is a Constant, Can’t be changed, but you can change the Data pointed to:

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    int daysInMonth = 30;
    int * const pDaysInMonth = &daysInMonth;
    *pDaysInMonth = 31; //This is ok, Data pointed to can be changed
    int daysInLunarMonth = 28;
    pDaysInMonth = &daysInLunarMonth; //This is shit, Cannot change address!

  • The second cace:

    Data pointed to is Constant, can’t be changed, but you can change the address which Pointer contained, and that means the Pointer can pointer to another variable:

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    int hoursInday = 24;
      const int* pointsToInt = &housday;
      int mouthInYear = 12;
      pointsToInt = &monthsInYear;
      *pointsToInt = 13; //Not OK! Cannot change data being pointed to
      int* newPointer = pointsToInt; //Not OK! cannot assign const to non-const

  • The third cace:

    Both Address that Pointer contains and Variable It pointer to is Constant, cannot be changed.(which is the most rigorous)

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    int hoursInDay = 24;
    const int* const pHousInDay = &hoursInDay;
    *pHoursInDay = 25; //Not OK! Cannot change data being pointed to
    int daysInMonth = 30;
    pHoursInDay = &daysInMonth; //Not OK! Cannot change address

Pass the pointer to the function.

When a pointer was used as a function argument, it’s important that Function only can change arguments that you hope to change.For example, when you passed a radius to calculate area of a circle, it can’t be changed. YOU CAN USE CONST.

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void CalcArea(const double* const ptrPi, const double* const ptrRadius, double* const ptr Area)//const pointer to const data, no changes allowed, can change data pointed to
{
  if(ptrPi && ptrRadius && ptrArea)
    *ptrArea = (*ptrPi) * (*ptrRadius) * (*ptrRadius);
  	//Pay attention to Product sign 'x' and indirect operater '*'
}

int main(){
  const double Pi = 3.14159265358979323846;
  double radius = 0;
  cin >> radius;
  
  double area = 0;
  CalcArea (&Pi, &radius, &area);
  //only area can be changed in func;
  return 0;
}
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