Review:   user-defined struct type and variables
  

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

 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);
 }

Pointer to a user-defined struct

Consider the following program with 2 struct BankAccount (local) variables: 

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

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


   john.balance = 500;


   mary.balance = 500;


   printf("j: %f  m:%f\n\n", john.balance, mary.balance); 
 }

Pointer to a user-defined struct

How to define a pointer variable p to a struct BankAccount typed variable: 

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

 int main( int argc, char *argv[] )
 {
   struct BankAccount john, mary, *p ;
           // p can store addr of struct BankAccount vars

   john.balance = 500;


   mary.balance = 500;


   printf("j: %f  m:%f\n\n", john.balance, mary.balance); 
 }

Pointer to a user-defined struct

&john and &mary are address value of struct BankAccount variables: 

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

 int main( int argc, char *argv[] )
 {
   struct BankAccount john, mary, *p ;
           // p can store addr of struct BankAccount vars

   john.balance = 500;
       &john 

   mary.balance = 500;
       &mary 

   printf("j: %f  m:%f\n\n", john.balance, mary.balance); 
 }

Pointer to a user-defined struct

So we can assign &john and/or &mary to p: 

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

 int main( int argc, char *argv[] )
 {
   struct BankAccount john, mary, *p ;
           // p can store addr of struct BankAccount vars

   john.balance = 500;
   p = &john; // Now *p ≡ john

   mary.balance = 500;
   p = &mary; // Now *p ≡ mary

   printf("j: %f  m:%f\n\n", john.balance, mary.balance); 
 }

Pointer to a user-defined struct

And we can use the expression (*p).balance to update their balance member variable: 

 #include <stdio.h>

 struct BankAccount {   
   int   ID;
   float balance;
 };

 int main( int argc, char *argv[] )
 {
   struct BankAccount john, mary, *p ;
           // p can store addr of struct BankAccount vars

   john.balance = 500;
   p = &john; // Now *p ≡ john
   (*p).balance = (*p).balance + 2000; 
   mary.balance = 500;
   p = &mary; // Now *p ≡ mary
   (*p).balance = (*p).balance + 9000; 
   printf("j: %f  m:%f\n\n", john.balance, mary.balance); 
 }

Why do you need ( ) in (*p).balance

  • There are 2 operators in the expression (*p).balance: 

          *   deference
    .   member access

  • The precedence of the member access operator . is higher than the deference operator *

    See: click here

  • So if you write: 

         *p.balance ≡ *(p.balance)

 Error:  p is not a struct BankAccount var !!    
         So p.balance is illegal !