P-3.2 Pointers, and What They Are For

Work through these

• A pointer holds an address, and that is the whole idea

  Taking the address of a variable gives you a number identifying where it lives, and dereferencing goes to that location to read or write. Everything else about pointers is consequence and notation.

  CS50x: Introduction to Computer Science · Course

• Passing an address so a function can change the caller's value

  Since arguments are copied, handing over an address is how a function reaches back into its caller. This is the reason pointers appear in the first week rather than the last.

• Pointer arithmetic moves by the size of the thing pointed at

  Adding one to a pointer advances it by one element rather than one byte, which is why the type matters so much. Getting this wrong reads memory that belongs to something else.

• Null pointers, and checking before you follow one

  A null pointer deliberately points nowhere and is how functions report that they have nothing to return. Following one stops the program, which is the good outcome compared with the alternative.

• Dangling pointers: an address that was valid a moment ago

  When storage is released or a function returns, addresses into it become meaningless while continuing to look ordinary. Reading through one sometimes works, which is what makes it so hard to find.