Deep understanding of pointers and pointers to C + + pointers _c language

Show a pointer to a pointer and a pointer that uses pointers to modify the pointers passed to the method to better use it. (The pointer is not a two-dimensional array.)

Why do they need to be used

When we pass a pointer as a parameter to a method, we actually pass a copy of the pointer to the method, or we can say that the pass pointer is the value of the pointer.

If we have a problem modifying the pointer within the method, the modification in the method is just the copy of the modified pointer rather than the pointer itself, and the original pointer retains the original

The value. Let's use the code below to illustrate the problem:

int m_value = 1;

void func (int *p)
{
  p = &m_value;
}

int main (int argc, char *argv[])
{
  int n = 2;
  int *PN = &n;
  cout << *pn << Endl;
  Func (PN);
  cout << *PN <<endl;
  return 0;
}

Check out the output.

The output is two 2

Using pointers to pointers

Show me how to use the pointer as a parameter

void func (int **p)
{
  *p = &m_value;

  You can also allocate memory according to your requirements
  *p = new int;
  **p = 5;
}

int main (int argc, char *argv[])
{
  int n = 2;
  int *PN = &n;
  cout << *pn << Endl;
  Func (&PN);
  cout << *PN <<endl;
  return 0;
}

Let's take a look at the func (int **p) method

P: Is a pointer pointer, where we will not modify it, otherwise we will lose this pointer to the pointer address

*p: Is the pointer that is being pointed to, is an address. If we modify it, we modify the contents of the pointer that is being pointed to. In other words, we are modifying the *PN pointer in the main () method

**p: Two-time dereference refers to the contents of *PN in the main () method

References to pointers

Look at the reference code of the pointer again

int m_value = 1;

void func (int *&p)
{
  p = &m_value;

  You can also allocate memory
  p = new int according to your requirements;
  *p = 5;
}

int main (int argc, char *argv[])
{
  int n = 2;
  int *PN = &n;
  cout << *pn << Endl;
  Func (PN);
  cout << *PN <<endl;
  return 0;
}

Look at the func (int *&p) method

P: is the reference to the pointer, the *PN in the main () method

*p: is the content that the PN points to in the main () method.

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