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Pointer to user-defined (struct) variables

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• Review: (1) reference (pointer) variables and (2) user-defined types (struct)
  • Recall:

    Reference data type: dataType* varName ; define a variable varName that can hold an address of a variable of type dataType (See: click here )

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  • Recall:

    Structure (record) definition: struct StructName { dataType1 varName1 ; dataType2 varName2 ; ... } define a data type with the name struct StructName that has the fields (components) varName1 of the data type dataType1, varName2 of the data type dataType2, ... (See: click here )

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• Pointer (reference) to struct variables
  • Address of a struct typed variable:

    Consider these 2 struct BankAccount variables: Observe that: The variable a is located at address 5000 in memory. The variable b is located at address 5012

  • Reference (pointer) variable to a struct BankAccount variable:

    Reference variable to a struct BankAccount variable = a variable that can contain an address of a struct BankAccount variable

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  • Syntax to define a reference (pointer) variable to a struct datatype:

    struct StructName *varName ;

    Meaning:

    defines a variable named varName that can store an address of a variable of the type struct StructName.

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  • How to use reference (pointer) veriables to struct typed variables:

    I will explain how to use reference (pointer) veriables to struct typed variables with an analogy - comparing its usage with how it was used in simple (int, double, etc) variables

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• How to use the reference operator "&" and dereference operator "*" on struct variables
  • Recall the reference operator:

    Reference operator & int a; &a is the address of the int variable a Analogously: (when applied to a struct) struct BankAccount b; &b is the address of the struct BankAccount b

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  • Recall the dereference operator:

    Dereference operator *: int a; int *p = &a; *p is an alias for the variable a Analogously: (when applied to a struct) struct BankAccount b; struct BankAccount *p = &b; *p is an alias for the variable b

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  • How to use reference variables to struct typed variables:

    Make a struct reference variable point (reference) to a struct typed variable: struct StructName *p ; // reference variable to a struct typed variable struct StructName a ; // A struct typed variable ... p = &a; // Make p point to the struct typed variable a Illustrated (using the BankAccount structure): Accessing component variables inside a struct using a reference typed variable: struct StructName *p ; // reference variable to a struct typed variable struct StructName a ; // A struct typed variable ... p = &a; // Make p point to the struct typed variable a // *p is now an alias for a !!! a.accNum access the accNum component in a (*p).accNum access the accNum component through p Illustrated (using the BankAccount structure)

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  • Syntax note:

    Fact: The   .   (member selection) operator has a higher priority than the * (dereference) operator !! See: click here Therefore: *p.accNum means: *(p.accNum) Which is in error, because * is applied to "p.accNum" (The variable accNum is NOT a pointer variable !!!) * should be applied to p !!! That's why we must use this syntax: (*p).accNum so that the * (dereference) operator is applied to p !

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  • Example program:

    struct BankAccount { int accNum; double balance; }; int main(int argc, char *argv[]) { struct BankAccount a, b; a.accNum = 123; a.balance = 1000.0; b.accNum = 444; b.balance = 3000.0; struct BankAccount *p; // Define a ref. var. to a struct p = &a; // Make p point to a printf("After p = &a\n"); printf("(*p).accNum = %d (*p).balance = %f\n", (*p).accNum, // = (*p) is a, so (*p).accNum is a.accNum (*p).balance); // = (*p) is a, so (*p).balance is a.balance p = &b; // Make p point to b printf("After p = &b\n"); printf("(*p).accNum = %d (*p).balance = %f\n", (*p).accNum, // = (*p) is b, so (*p).accNum is b.accNum (*p).balance); // = (*p) is b, so (*p).balance is b.balance }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc struct-ref1.c To run:          ./a.out