Structures:   user-defined data types in C

  • Structure:

      • A structure in a C program is a group of related variables

    Example:

      • A BankAccount structure may consists of the related variables ID and balance

     

     

     

     

     

Defining a C struct

  • Syntax to define a struct (user defined) data type in C: 

        struct structureName 
  {
     variable definitions;
  } [varname1, varName2] ;  // Optional elements

  • Example: 

        struct BankAccount {                         
     int   ID;
     float balance;
  } ;

  • Important note:

      • A struct data type definition is in the global scope

      • struct data type definitions must be placed outside every function !

Comparing C's struct with Java's class construct

  • The struct definition in C is similar to a class definition in Java:

      C's struct Java's class 
       struct BankAccount      
 { 
    int   ID;
    float balance;                  
 }






       public class BankAccount    
 {
    public int   ID;
    public float balance;

    public void deposit(double x)
    {
        balance += x;
    }
 }

  • Differences:

      1. No access protection of member fields in a C's struct

      2. A struct does not contain any (member) function definitions

Defining (user-defined) struct typed variables

  • You can define struct typed variable(s) after defining the struct data type

    Rule:

      • The struct (data type) definition must preceed any variable definition of that structure data type !!!

  • Syntax to define a struct typed variable in C: 

        struct structName  varName ;

    Example: 

        struct BankAccount            // (1) Must define struct first
  {   
     int   ID;
     float balance;
  } ;  

  struct BankAccount john, mary; // (2) Then define variable(s)

Using a struct typed variable
 

  • A struct variable in C is similar an object in Java

  • Recall:

      • In Java, you can access a specific member field using the member selection operator (.): 

            objectRef.memberName

  • C has an similar syntax to access a field in a struct variable x 

          x.fieldName

    (A field in a C struct is similar to a member in a Java object)

Using a struct typed variable

Example program: defining and using struct typed variables 

 #include <stdio.h>

 // (1) You must first define the struct definition 
 struct BankAccount {   
   int   ID;
   float balance;
 };

 // (2) Afterwards, you can define struct typed variables
 struct BankAccount mary;    // Global struct variable

 int main( int argc, char *argv[] )
 {
   struct BankAccount john;  // Local struct variable

   mary.balance = 900;
   john.balance = 500;
   printf("%f\n\n", mary.balance + john.balance);
 }

DEMO: demo/C/set2/struct1.c

Fundamental difference between struct variables in C and class variables in Java

  • There are 2 (two) fundamental differences between C's struct and Java's objects:

    1. How to find an object/struct:

      • Java uses a reference variable to record the location of an object

      • C uses a symbolic constant to record the location of an struct

    2. The meaning of the (variable) name of an object/struct variable: 

            Java                               C
  ---------------------------  --------------------------
  myClass x;                    struct myStruct x;

  x = the reference to          x = the entire struct
      an object                     (="object")

Fundamental difference between struct variables in C and class variables in Java

  • Class variables in Java are reference variables to objects (that are created by new): 

     BankAccount a, b; // Define 2 reference variables

  • The BankAccount objects are created using: 

        a = new BankAccount( );
    b = new BankAccount( );
    Result:

Fundamental difference between struct variables in C and class variables in Java

  • The variable name a in Java is a reference (= address): 

      b = a;   // Copies a reference (address)

    will copy the reference (= address) in a into the (reference) variable b

  • Result:   b will now reference (= point) to the same object as a:

    I.e.:   b is now an alias of a:

Fundamental difference between struct variables in C and class variables in Java

Example Java program that shows b = a makes b an alias of a 

public class struct_copy
{
   public static BankAccount a;

   public static void main(String[] argv)
   {
      BankAccount b;

      a = new BankAccount( );  // Create an object
      b = new BankAccount( );  // Create another object

      a.ID  = 123;
      a.balance = 1000.0;

      b.ID  = 444;
      b.balance = 3000.0;

      System.out.printf("a = (%d, %f)\n", a.ID, a.balance);   // a and b are
      System.out.printf("b = (%d, %f)\n\n", b.ID, b.balance); // diff objs

      b = a;  // b is now alias of a

      a.balance = 999999.0;

      System.out.printf("a = (%d, %f)\n",   a.ID, a.balance); // 123, 999999.0
      System.out.printf("b = (%d, %f)\n\n", b.ID, b.balance); // 123, 999999.0
   }

DEMO: demo/C/set2/struct_copy.java

Fundamental difference between struct variables in C and class variables in Java

  • Struct variables in C are not reference variables but actual "objects": 

     struct BankAccount a, b; // Define 2 BankAccount "objects"

  • This diagram shows the result of the above struct definition:

    The variable names a and b are symbolic constants for the memory locations (= addresses) where the BankAccount "objects" (= structs) are stored

Fundamental difference between struct variables in C and class variables in Java

  • The variable name a in C is an (entire) struct (="object"): 

      b = a;   // Copies an entire object

    will copy the struct (= "object") in a into the struct (= "object") b

  • Result:   b will be (real) copy of a:

    I.e.:   b is not an alias of a !!!

Fundamental difference between struct variables in C and class variables in Java

Example C program that shows b = a will copies the values in a over to b: 

struct BankAccount
{  
   int    ID;
   double balance;
};

struct BankAccount a;

int main(int argc, char *argv[])
{
   struct BankAccount b;

   a.ID  = 123;
   a.balance = 1000.0;

   b.ID  = 444;
   b.balance = 3000.0;

   printf("a = (%d, %f)\n", a.ID, a.balance);   // a and b are
   printf("b = (%d, %f)\n\n", b.ID, b.balance); // diff objs

   b = a;  // Copies content of a to b

   a.balance = 999999.0;

  printf("a = (%d, %f)\n",   a.ID, a.balance); // 123, 999999.0
  printf("b = (%d, %f)\n\n", b.ID, b.balance); // 123, 1000.0  
}

DEMO: demo/C/set2/struct-copy.c

A quick way to define global struct typed variables

  • You may include some variable names when you define a struct: 

      struct BankAccount 
  {   
     int   ID;
     float balance;
  } john;             // Also defines a (global) "struct" variable 
                      // along with the "struct" data type

  • Note:

    • You cannot define a local struct variable in this manner
      (because the struct definition must be global)

    • You can only define global struct variables this way.

  • This feature is handy if your program only uses a few global struct variables of the specific type....