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Pointer to user-defined (struct) variables

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• Pointer to user-defined data type
  • You can define a reference (= pointer) to any data type, including to user-defined type (= struct)

    Previously, we disccussed user-defined data type (= struct) on this webpage: click here

    For review, here is an example program on how to define and use a struct typed variable:

    #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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  • Pointer variable to a user-defined type (= struct):

    You can define a reference (= pointer) variable to a user-defined type (= struct) as follows: struct BankAccount *p; // Defines variable p as a pointer to // any struct BankAccount variable You can make p point to the BankAccount object a using this assignment: struct BankAccount a; p = &a; // &a = address of the BankAccount object a // p points to a // The expression *p is now an alias for a This diagram depicts what happens inside the variables after the assignment p = &a (assuming that the object a was stored at address 4000 in memory):

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  • This program shows you how to define and use a reference variable of the user-defined type BankAccount:

    #include <stdio.h> /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; struct BankAccount a; int main(int argc, char *argv[]) { struct BankAccount b; struct BankAccount *p; // Reference variable to type "struct BankAccount" p = &a; // Now: *p is alias for a (*p).accNum = 123; // This statement is equal to: a.accNum = 123; (*p).balance = 1000.0; // This statement is equal to: a.balance = 1000.0; p = &b; // Now: *p is alias for b (*p).accNum = 444; // This statement is equal to: b.accNum = 444; (*p).balance = 9999.0; // This statement is equal to: b.balance = 9999.0; printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); printf("b.accNum = %d b.balance = %f\n", b.accNum, b.balance); }

    Output:

    cs255-1@aruba (5584)> a.out a.accNum = 123 a.balance = 1000.000000 b.accNum = 444 b.balance = 9999.000000

    Important note:

    We must use (*p).accNum because the member selection operator "." has a higher precedence (= priority) that the deference operator "*" See the operator precedence table: click here In other words: *p.accNum is processed as: *( p.accNum ) which is wrong because p is not a BankAccount typed variable (In order to write p.accNum as a correct C language expression, the variable p must have the type: struct BankAccount. The variable p has the type: struct BankAccount * !!!)

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

    How to run the program:

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