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The short-hand operator "->"

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• Prelude: syntactical sugar
  • Syntactical sugar:

    Syntactical sugar = an programming language construct that look nice ("sweeter") but does not add more features (sugar has no nutritional value...) Wikipedia page on syntactical sugar: click here

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  • The C programming language has a lot of syntactical sugars....

    Here a some that you have already learned:

    Ordinary way | "Nicer looking" way ----------------+----------------------- i = i + 1 | i++ // The ++ is a new operator !!! i = i - 1 | i-- i = i + 7 | i += 7 .... | ....

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  • We will now introduce a very nicely looking operator that is used extremely often by C programmer when they use reference variables to struct typed variables

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• The "->" operator: short hand for the expression "(*p)."
  • Fact:

    C programs often use pointer to structures

    Therefore:

    A C program will often use: (*p).VarName

    This occur so often than C has introduced a short-hand notation !!!

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  • The -> operator:

    The expression p->varName // p must be defined as: struct StructName* p ; is a short hand for: (*p).varName

    Illustrated (using the BankAccount struct):

    The original syntax to access member variables in a struct through a pointer variable: p   points to a struct variable Then:   *p   is a struct variable Therefore:   (*p).varName   selects a member variable inside a struct variable The new syntax to access member variables in a struct through a pointer variable: You can visualize the -> operator as an arrow from the pointer variable to a member variable inside the struct:

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  • Example program:

    #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); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/ref-struct2.c

    How to run the program:

    To compile:   gcc ref-struct2.c          To run: ./a.out

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  • Why is it called a short hand:

    The expression (*p).accNum contains 2 operations: * (de-reference operator) . (the member selection operator)          In contrast, the expression: p->accNum contains only 1 operation: -> (de-reference plus member selection operator)