Lecture 12: Pointers

Where we left off

#include <fstream>

int main(int argc, char *argv[]) {
    ofstream output(argv[1]);
    output << "Writing to a file!\n";
    output.close();
    return 0;
}

Today’s topics

• A bit of midterm info
• Intro to pointers
• Assigning and dereferencing pointers

Textbook Chapter 9, kinda

Midterm Info

• Topics up to and including Pointers (this week)
• Format: multiple choice, short answer, tracing, coding
• No cheat sheet, but I will provide the operator precedence table
• Expect coding questions similar to assignments 1 and 2, plus conceptional questions about memory allocation

That mysterious issue from last lecture

• In the copy_and_meow function, I had the following loop:

  char line[256];
  while (in.getline(line, 256)) {
       str_replace(line, "now", "meow");
       out << line << endl;
  }
  

• Turns out that my text file had a line longer than 256 characters
• I RTFM and realized that while the first 255 chars were read into line, the failbit was set on in and the loop never executed
• Lesson: choose your buffer size wisely!

And now, pointers!

• Pointers are a powerful and confusing feature of C++
• They allow for dynamically sized arrays, linked data structures, and more
• It’s also how pass-by-reference works in C++
• We’ve been using pointers already!
  • void add_one(int arr[], int size)
  • Passing an array to this function passes a pointer to the first element of the array - this is why putting a size in the [] doesn’t matter

Memory and Addresses

• Memory is a sequence of bytes (8 bits), the smallest addressable unit
• Declaring a variable allocates enough memory to store the value, and also allows us to reference the location by name
• The address of a variable is the location in memory where it is stored

Allocating Memory: Example

int main() {
    int i = 42;
    int j;
    char c = 'K';
    double d = 3.14159;
    // ...
    return 0;
}

The memory addresses are integers, though usually hexadecimal (base 16)

The pointer type

• A pointer is a variable that stores the address of another variable
• The value of a pointer doesn’t make sense on its own
• Memory addresses are integers, but pointers are a specific type, such as:
  • Pointer to an int
  • Pointer to a char
  • Pointer to a BillInfo struct

Declaring pointers

• Passing by reference uses &, but this is the address-of operator
  • int &x = y; is a compile time error
• The actual syntax:

  type *variable_name;
  

  where type is the type of the variable being pointed to
• Example:

  int x; // a normal integer variable
  int *p; // a pointer to an integer
  

The * is not part of the type!

While C++ allows the * to be placed anywhere between the type and the variable, you have to be very careful:

char *cptr; // pointer to a char
char* cptr2; // also a pointer to a char

int *ptr1, *ptr2; // Two pointers to ints
int* ptr3, ptr4; // ptr3 is a pointer to an int, ptr4 is an int

• Keeping the * next to the variable name helps to keep things straight

What happens when we declare a pointer?

• Like other variable declarations:
  • Memory is allocated
  • The value is uninitialized (random garbage)
• Regardless of the type, memory allocated to a pointer is the size of an int
• The random garbage may or may not point to a valid memory address

Side tangent: Segmentation fault

• A segmentation fault or “segfault” might happen if you:
  • Try to access memory that doesn’t belong to you
  • Try to write to read-only memory
  • Try to get the value of unallocated memory
• Basically, any time you mess with memory that isn’t your own, you might see:

  Segmentation fault (core dumped)
  

• “Core dump” is a reference to old-school magnetic memory cores
• You can backtrace in gdb to find the offending code

Initializing pointers

• We can initialize pointers to point to a specific memory address:

  int *p = 0x7ffeeb6b4a4c;
  

• But this is pretty much useless
• The only useful predefined pointer value is NULL, which is falsy

  int *iptr = NULL;
  char *cptr = NULL;
  

The & operator

• We’ve already seen the address-of operator, used for pass-by-reference
• Recalling the diagram to the right:
  • &i evaluates to 000
  • &j evaluates to 004
  • &c evaluates to 008
  • &d evaluates to 012
• We now have valid addresses that can be assigned to pointer variables!

Assigning addresses to pointers

Consider the following:

int i = 42;
int *iptr = &i;
iptr = &j;

What just happened??

Let’s draw a diagram!

int *iptr2;
iptr2 = iptr;

int x = 24;
int *iptr = &x;
char c;
char *cptr = &c;