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Structures: user-defined types

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• Defining structures
  • Structure:

    A structure in C is a group of related variables The structure construct is the user-defined data type in the C programming language

  • Comparable concept in Java:

    A structure in C is like a class in Java containing: Only public variables            and            without any methods defined inside the class

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  • Defining a structure (= user-defined data type)

    Defining a structure = tell (convey) to the C compiler the make up (components) of the structure (= user-defined data type)

  • Syntax to define a structure:

    struct StructName { dataType1 member1Name ; dataType2 member2Name ; ... };

    Example:

    /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; };

    Note:

    A structure definition must appear outside every function (I.e., a structure is a global C programming language construct - it cannot appear inside a function definition) A structure definition is similar to a class definition in Java that does not contain any methods Futhermore: all member variables are public So an semi-equivalent construct in Java for the above struct definition is: public class BankAccount { public int accNum; public double balance; }

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• Defining structure variables
  • After defining a structure, you can define program variables of that specific structure (= user-defined data type).

    Important note:

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

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  • Syntax to define structure variables:

    (The structure must have been define beforehand !!!) struct StructName varName1 [, varName2, ...] ;

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  • Example:

    /* --------------------------------------------- Structure definition This MUST preceed any variable definition of this data type !!! --------------------------------------------------------------- */ struct BankAccount { int accNum; double balance; }; struct BankAccount a; // Defines a global structure variables int main(int argc, char *argv[]) { struct BankAccount b; // Defines a local structure variables .... }

    Result of a structure variable definition:

    The following figure shows the effect of the structure variable definition in the example: Explanation: A structure variable definition will allocate (= reserve memory space) all components (variables) that was specified in the structure definition clause. The variable names (a and b) will be associated with the starting location of the allocated (= reserved) memory cells For example, in the above figure: The identifier b (actually: the expression &b) is equated to the value 5000 (= starting address of the components in the structure b) The identifier a (actually: the expression &a) is equated to the value 5016 (= starting address of the components in the structure a)

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• A note on the difference between C and Java
  • C is very "manual":

    The programmer must make sure that the type information (such as the struct BankAccount definition is available before the definition of variables of that given data type

    Consequently, a good C programmer must understand how a compiler works

    (That's why I have injected information on how the C compiler works in the lecture material)

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  • In contrast, Java tries very hard to make the programmer's life easy by doing thing for him/her....

    Example:

    If Java detected that a unknown data type is used, the Java compiler will try to find the Java program file that contains the definition and include the information automatically

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  • This Java program illustrate this automatic search

    I will do the demo in class

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Java/main.java + BankAccount.java

    How to run the program:

    To compile:   javac main.java You will see that the Java compiler has also compiler BankAccount.java

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• Using (accessing) structure variables
  • Fact:

    Using structure variables in C is very similar to using object variables in Java

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  • Syntax to use (= access) a specific component of a structure variable:

    structVarName.componentName

    Example:

    #include <stdio.h> /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; struct BankAccount a; // Global variable int main(int argc, char *argv[]) { struct BankAccount b; // Local variable a.accNum = 123; // Access member fields a.balance = 1000.0; b.accNum = 444; b.balance = 3000.0; printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); printf("b.accNum = %d b.balance = %f\n", b.accNum, b.balance); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/struct1.c

    How to run the program:

    To compile:   gcc struct1.c          To run: ./a.out

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• Important difference: copying struct variables
  • Due to the difference in the meaning when you define a structure (= object) variable, the assignment statement with structure (= object) variable have different effects:

    In Java, the variable definition: BankAccount a, b; // Define 2 reference variables to BankAccount objects You still need to create the BankAccount objects using: a = new BankAccount( ); b = new BankAccount( ); This diagram shows a sample result when the Java program has executed the above (2) program constructs: The following assignment statement in Java: b = a; // Copies a reference (address) will have this result: b will reference (= point) to the same object as a When you update a.balance, the variable b.balance will now also change because a and b points to the same object In contrast, in C, the variable definition: struct BankAccount a, b; // Define 2 BankAccount objects !!! This diagram shows a sample result when the C program has executed the above program construct: In contrast, the following assignment statement in C: b = a; // Copies an object !!! will have this result: We will copy all fields in structure (= object) a into the structure (= object) b When you update a.balance, the variable b.balance will not change because a and b are different object

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  • Java and C Programs to show the difference in the above behavior:

    struct_copy.java

    struct-copy.c

    class BankAccount { int accNum; double balance; }; public class struct_copy { public static BankAccount a; public static void main(String[] argv) { BankAccount b; a = new BankAccount( ); // Create the objects b = new BankAccount( ); a.accNum = 123; a.balance = 1000.0; b.accNum = 444; b.balance = 3000.0; System.out.printf("a = (%d, %f)\n", a.accNum, a.balance); System.out.printf("b = (%d, %f)\n\n", b.accNum, b.balance); /* ======================== Copy object reference ======================== */ b = a; System.out.printf("a = (%d, %f)\n", a.accNum, a.balance); System.out.printf("b = (%d, %f)\n\n", b.accNum, b.balance); /* ============================================ Proof that a and b are the SAME object ============================================ */ a.balance = 999999.0; System.out.printf("a = (%d, %f)\n", a.accNum, a.balance); System.out.printf("b = (%d, %f)\n\n", b.accNum, b.balance); } }

    #include <stdio.h> /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; struct BankAccount a; int main(int argc, char *argv[]) { struct BankAccount b; a.accNum = 123; a.balance = 1000.0; b.accNum = 444; b.balance = 3000.0; printf("a = (%d, %f)\n", a.accNum, a.balance); printf("b = (%d, %f)\n\n", b.accNum, b.balance); /* ======================== Copy struct ======================== */ b = a; printf("a = (%d, %f)\n", a.accNum, a.balance); printf("b = (%d, %f)\n\n", b.accNum, b.balance); /* ============================================ Proof that a and b are different objects ============================================ */ a.balance = 999999.0; printf("a = (%d, %f)\n", a.accNum, a.balance); printf("b = (%d, %f)\n\n", b.accNum, b.balance); }

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    The output of the Java program struct_copy.java is:

    a = (123, 1000.000000) b = (444, 3000.000000) a = (123, 1000.000000) b = (123, 1000.000000) After we update a.balance = 999999.000000: a = (123, 999999.000000) b = (123, 999999.000000) // b.balance is also changed !!

    This shows that a and b points to the same object

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    The output of the C program struct-copy.c is:

    a = (123, 1000.000000) b = (444, 3000.000000) a = (123, 1000.000000) b = (123, 1000.000000) After we update a.balance = 999999.000000: a = (123, 999999.000000) b = (123, 1000.000000) // b.balance is unchanged !!

    This shows that a and b are different objects