Pointer expressions: * ptr ++, *++ptr and ++* ptr

In C or C++ programming, pointer expressions *ptr++, *++ptr, and ++*ptr are crucial as they have distinct meanings and uses.

1. *ptr++

This expression combines two operations: pointer increment (ptr++) and dereferencing (*). Due to operator precedence rules in C and C++, ptr++ is evaluated before *, but because ++ is a postfix operator, its effect is deferred until after the dereference operation.

• Purpose: First obtain the current value pointed to by ptr, then increment ptr to the next memory location.

• Use Case Example: This is commonly used for iterating through elements in arrays or strings. For instance, when traversing a string and printing each character, you can use a loop like:

  c
  char* s = "Hello";
  while(*s) {
      printf("%c ", *s++);
  }

2. *++ptr

This expression also involves dereferencing and pointer increment, but here ++ is a prefix operator. Prefix increment has higher precedence than dereferencing.

• Purpose: First increment ptr to the next memory location, then dereference to obtain the new value.

• Use Case Example: This is useful if you want to skip the first element and process from the second element of an array:

  c
  int arr[] = {1, 2, 3, 4};
  int* ptr = arr;
  printf("%d ", *++ptr);  // Output: 2

3. ++*ptr

In this expression, dereferencing (*) has higher precedence than prefix increment (++).

• Purpose: First dereference to obtain the value pointed to by ptr, then increment that value by 1.

• Use Case Example: This is very useful when you need to increment the value pointed to by the pointer without moving the pointer itself:

  c
  int val = 10;
  int* ptr = &val;
  ++*ptr;
  printf("%d", *ptr);  // Output: 11

In summary, although these three pointer expressions differ only in the order of operators, their effects and applicable scenarios are significantly different. Understanding these distinctions is crucial for writing correct and efficient pointer manipulation code.