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Class 3

An object is a chunk of memory that holds some value from the possible set of values for the object's type

A variable is a name that refers to an object

int x = 3;
  • value is 3
  • type is int
  • variable is x

Memory

  • Objects are stored in memory, which is a bunch of storage locations with addresses from 0 to a very large number

Binary

  • Base 2

  • Adding zeroes to the left doesn't change the value

Example

In decimal: 1492

In binary: 1011

Hex

  • Groups of 4 bits called nibbles, starting at the least significant bit (LSB)

  • Each 4-bit group as a value from 0 to 15

  • Values 10 to 15 written as A to F

  • Base 16

Example

0111 0100 1001 1111

749F

Conversion

Decimal to binary

  1. Divide by 2 until the result is 0
  2. The remainder is the next bit, starting with the LSB
  3. Last bit is the MSB

Example

Value Result Remainder
957 478 1
478 239 0
...
1 0 1

957 decimal = 11 1011 1101 binary = 3BD hex

Decimal to hex

  1. Convert decimal to binary
  2. Convert binary to hex

More memory

Address example

int x = 3;

The address is 0x804240c0 (2151825600 in decimal)

0x tells the computer the number is in hex

The variable is x

The value is 3

Changing the value of x would only change the value, not the address

  • C++ uses value semantics, which means initialization and assignment involve copying the value from one object to another

Initialization

Giving an object an inital value when it is created

int x = 3;

Assignment

Overwrite old value of an object with new value

x = 3;

The value of the above assignment is 3, so the following expression is valid.

a = x = 3;
  • To get the address of a variable, use the address of operator &

Address of example

cout << &x; //0x804240c0
  • Don't confuse the address of operator with a reference type

Reference-to-int examples

int &r = x;

r is a variable whose type is reference-to-int

The variable r is a new name for x

int swap (int &a, int &b);

a and b are variables whose type is reference-to-int

  • address of operator returns a pointer type
int x = 7;
cout << &x; //0x804240c0
int *p = &x;