Arrow operator - usage in C

Question

I am reading a book called  Teach Yourself C in 21 Days   I have already learned Java and C  so I am moving at a much faster pace   I was reading the chapter on pointers and the - gt   arrow  operator came up without explanation  I think that it is used to call members and functions  like the equivalent of the    dot  operator  but for pointers instead of members   But I am not entirely sure   Could I please get an explanation and a code sample

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Yes  that s it   It s just the dot version when you want to access elements of a struct class that is a pointer instead of a reference   struct foo     int x    float y      struct foo var  struct foo  pvar  pvar   malloc sizeof pvar     var x   5    amp var - gt y   14 3  pvar- gt y   22 4    pvar  x   6    That s it

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include lt stdio h gt   int main         struct foo               int x          float y        var1      struct foo var      struct foo  pvar       pvar    amp var1         if pvar    amp var  it directly         takes values stored in var  and if give          new  gt  values like pvar- gt x   6  pvar- gt y   22 4          it modifies the values of var          object  so better to give new reference         var x   5        amp var - gt y   14 3      printf   i -   02f n   var x    amp var - gt y        pvar- gt x   6      pvar- gt y   22 4      printf   i -   02f n   pvar- gt x  pvar- gt y        return 0

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I d just add to the answers the  why       is standard member access operator that has a higher precedence than   pointer operator   When you are trying to access a struct s internals and you wrote it as  foo bar then the compiler would think to want a  bar  element of  foo   which is an address in memory  and obviously that mere address does not have any members   Thus you need to ask the compiler to first dereference whith   foo  and then access the member element    foo  bar  which is a bit clumsy to write so the good folks have come up with a shorthand version  foo- gt bar which is sort of member access by pointer operator

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The - gt  operator makes the code more readable than the   operator in some situations   Such as   quoted from the EDK II project   typedef EFI STATUS  EFIAPI  EFI BLOCK READ     IN EFI BLOCK IO PROTOCOL           This    IN UINT32                         MediaId    IN EFI LBA                        Lba    IN UINTN                          BufferSize    OUT VOID                           Buffer        struct  EFI BLOCK IO PROTOCOL               The revision to which the block IO interface adheres  All future       revisions must be backwards compatible  If a future version is not       back wards compatible  it is not the same GUID          UINT64              Revision              Pointer to the EFI BLOCK IO MEDIA data for this device          EFI BLOCK IO MEDIA   Media     EFI BLOCK RESET     Reset    EFI BLOCK READ      ReadBlocks    EFI BLOCK WRITE     WriteBlocks    EFI BLOCK FLUSH     FlushBlocks        The  EFI BLOCK IO PROTOCOL struct contains 4 function pointer members   Suppose you have a variable struct  EFI BLOCK IO PROTOCOL   pStruct  and you want to use the good old   operator to call it s member function pointer  You will end up with code like this     pStruct  ReadBlocks    arguments      But with the - gt  operator  you can write like this   pStruct- gt ReadBlocks    arguments       Which looks better

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foo- gt bar is only shorthand for   foo  bar  That s all there is to it

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Well I have to add something as well  Structure is a bit different than array because array is a pointer and structure is not  So be careful      Lets say I write this useless piece of code    include  lt stdio h gt   typedef struct          int km          int kph          int kg        car   int main void        car audi    12000  230  760       car  ptr    amp audi       Here pointer ptr points to the address     of the structure variable audi but beside address structure also has a chunk of data      The first member of the chunk of data has the same address than structure itself and you can get it s data by only dereferencing a pointer like this  ptr  no braces     But If you want to acess any other member than the first one  you have to add a designator like  km   kph   kg which are nothing more than offsets to the base address of the chunk of data      But because of the preceedence you can t write  ptr kg as access operator   is evaluated before dereference operator   and you would get   ptr kg  which is not possible as pointer has no members  And compiler knows this and will therefore issue an error e g    error     ptr    is a pointer  did you mean to use    - gt        printf   d n    ptr km     Instead you use this   ptr  kg and you force compiler to 1st dereference the pointer and enable acess to the chunk of data and 2nd you add an offset  designator  to choose the member   Check this image I made     But if you would have nested members this syntax would become unreadable and therefore - gt  was introduced  I think readability is the only justifiable reason for using it as this ptr- gt kg is much easier to write than   ptr  kg   Now let us write this differently so that you see the connection more clearly    ptr  kg      amp audi  kg   audi kg  Here I first used the fact that ptr is an  address of audi  i e   amp audi and fact that  reference   amp  and  dereference    operators cancel eachother out

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foo- gt bar is equivalent to   foo  bar  i e  it gets the member called bar from the struct that foo points to

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include lt stdio h gt  struct examp  int number     struct examp a  b  amp a  enter code here  main     a number 5     a number b- gt number   b  number produces same output  b- gt number is mostly used in linked list      printf   d  n  d  n  d  a number b- gt number   b  number       output is 5           5 5

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struct Node       int i      int j     struct Node a   p    amp a    Here the to access the values of i and j we can use the variable a and the pointer p as follows  a i    p  i and p- gt i are all the same   Here   is a  Direct Selector  and - gt  is an  Indirect Selector

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I had to make a small change to Jack s program to get it to run  After declaring the struct pointer pvar  point it to the address of var  I found this solution on page 242 of Stephen Kochan s Programming in C    include  lt stdio h gt   int main       struct foo         int x      float y          struct foo var    struct foo  pvar    pvar    amp var     var x   5      amp var - gt y   14 3    printf   i -   02f n   var x    amp var - gt y     pvar- gt x   6    pvar- gt y   22 4    printf   i -   02f n   pvar- gt x  pvar- gt y     return 0      Run this in vim with the following command     gcc -o var var c  amp  amp    var   Will output   5 - 14 30 6 - 22 40

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a- gt b is just short for   a  b in every way  same for functions  a- gt b   is short for   a  b

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Dot is a dereference operator and used to connect the structure variable for a particular record of structure   Eg    struct student             int s no        Char name           int age        s1 s2   main               s1 name        s2 name          In such way we can use a dot operator to access the structure variable

[c] Arrow operator (->) usage in C

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