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typedef: give an existing type another name...

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• typedef: giving a user-defined data type a name
  • The typedef clause in the C programming language:

    A typedef definition is an single word alias for a user-defined data type (such as a struct)

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  • Syntax of the typedef clause

    typedef Any-data-type Single-word-alias ;

    Example:

    typedef int my_int ;

    This typedef clause will define my_int as an alias for int

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  • Example program:

    typedef int my_int; int main(int argc, char *argv[]) { int a; my_int b; // b is actually an int !!! a = 123; printf("a = %d\n", a); /* ================================================== If my_int is really a NEW type, then you CANNOT assign an int to an my_int variable !!! =================================================== */ b = a; // Proof that my_int is no different than int printf("b = %d\n", b); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/typedef3.c

    How to run the program:

    To compile:   gcc typedef3.c          To run: ./a.out

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• Common use of typedef
  • The most common usage of typedef is to shortened a struct data type defined by the programmer.

    It is common practice among C programmers to use the name: structureName_t for the typedef name

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  • Example:

    /* ----------------------------------------------- The struct definition and the typedef should be placed inside a header file and included by the main program file.... ----------------------------------------------- */ struct BankAccount { int accNum; double balance; }; typedef struct BankAccount BankAccount_t ; // new name (alias) int main(int argc, char *argv[]) { BankAccount_t b; // We can use the typedef-ed name struct BankAccount a; // AND you can STILL use the original method a.accNum = 123; a.balance = 4000; printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); /* =============================================== Proof that they are the same type of variable ================================================ */ b = a; // You can assign them to each other ! printf("b.accNum = %d b.balance = %f\n", b.accNum, b.balance); }

    Result:

    a.accNum = 123 a.balance = 4000.000000 b.accNum = 123 b.balance = 4000.000000

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/typedef1.c

    How to run the program:

    To compile:   gcc typedef1.c          To run: ./a.out

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• Alternative syntax of typedef
  • The typedef clause can be combined with a structure definition and the shortened form has the following syntax:

    typedef struct/union definition new-name ;

    Example:

    typedef struct BankAccount { int accNum; double balance; } BankAccount_t; int main(int argc, char *argv[]) { struct BankAccount a; // You STILL can use struct BankAccount ! BankAccount_t b; a.accNum = 123; a.balance = 4000; printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); b = a; printf("b.accNum = %d b.balance = %f\n", b.accNum, b.balance); }

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

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc typedef2.c To run:          ./a.out

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• Summary
  • Summary:

    typedef gives a new name to an existing data type...