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The reference data type and reference variables

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• What make C special ?
  • Historical fact:

    C was designed by Kernighan and Ritchie to help them write (= implement) the UNIX operating system Even today, the UNIX operating system is written (mostly, because some modules must be written in assembler code) in the C programming language

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  • What make the C programming language so special:

    C has a very dangerous data type: The reference data type         that no other (popular) programming language has (as far as I can tell).

    The reference data type allows the program reference (= point) to any where in the computer memory and access data stored at that memory location !!!

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• Reference data type

  • Reference:

    Reference = an address in memory A reference is also known as a pointer Example: The address 4 points to memory location 4 in the memory

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  • Reference (or pointer) data type:

    Reference data type = a data type that consists of all address of the memory

    Consequently:

    A variable of the reference data type can store an address of the memory (And C has operators that manipulate this address !!!! Will be discussed later)

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• Reference variables.... or pointer variables (or "pointers")

  • Reference variable:

    Reference variable = a variable that can contain a reference (= address) Typically: Reference variable = a variable that contains the address of another variable

    A reference variable is often called a pointer variable

    I will often use both terminology interchangeably !!!

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• Defining a reference variable
  • Syntax:

    dataType *varName ;

    Meaning:

    The variable varName can contain the address of a variable of the data type dataType

    Important note:

    The symbol * is also used a the multiply operator The * multiply operator must be used in a statement !!! When the C compiler see the * symbol used in a variable definition, it know that this * symbol is used to define an address typed variable !!!

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  • Reference variable:

    We call a variable that holds an address value: a reference variable

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  • Example:

    int *p ; // p can contain the address of an int typed variable float *q ; // q can contain the address of a float typed variable

    So p and q are reference variables.

    Note:

    Because p and q points to different types of variables (p points to an int typed variable while q points to a float type variable, the data types of p and q are different

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• Mixed data type definition
  • Important note:

    You will often see C programs with mixed data type variable definitions. Example: the following variable definition int a, *p; 2 variables: Variable a is an int typed variable Variable p is an int reference typed variable So the single definition will define variables of different data types !!!

    (Persoanlly, I wish C had a cleaner way to define the reference typed variable...)