Why does the arrow - operator in C exist

Question

The dot     operator is used to access a member of a struct  while the arrow operator  - gt   in C is used to access a member of a struct which is referenced by the pointer in question   The pointer itself does not have any members which could be accessed with the dot operator  it s actually only a number describing a location in virtual memory so it doesn t have any members   So  there would be no ambiguity if we just defined the dot operator to automatically dereference the pointer if it is used on a pointer  an information which is known to the compiler at compile time afaik    So why have the language creators decided to make things more complicated by adding this seemingly unnecessary operator  What is the big design decision

User · Answer

C also does a good job at not making anything ambiguous.

Sure the dot could be overloaded to mean both things, but the arrow makes sure that the programmer knows that he's operating on a pointer, just like when the compiler won't let you mix two incompatible types.

User · Answer

Beyond historical  good and already reported  reasons  there s is also a little problem with operators precedence  dot operator has higher priority than star operator  so if you have struct containing pointer to struct containing pointer to struct    These two are equivalent         a  b  c  d  a- gt b- gt c- gt d   But the second is clearly more readable  Arrow operator has the highest priority  just as dot  and associates left to right  I think this is clearer than use dot operator both for pointers to struct and struct  because we know the type from the expression without have to look at the declaration  that could even be in another file

User · Answer

Structure in C First you need to declare your structure  struct mystruct   char element 1   char element 2     Instantiate C structure Once you declared your structure   you can instantiate a variable that has as type your structure using either  mystruct struct example   or   mystruct  struct example   For the first use case you can access the varaiable eleemnet using the following syntax  struct example element 1   5  For the second use case which is having a pointer to variable of type your structure  to be able to access the variable structure you need an arrow  struct example- gt element 1   5

User · Answer

I ll interpret your question as two questions  1  why - gt  even exists  and 2  why   does not automatically dereference the pointer  Answers to both questions have historical roots   Why does - gt  even exist   In one of the very first versions of C language  which I will refer as CRM for  C Reference Manual   which came with 6th Edition Unix in May 1975   operator - gt  had very exclusive meaning  not synonymous with   and   combination  The C language described by CRM was very different from the modern C in many respects  In CRM struct members implemented the global concept of byte offset  which could be added to any address value with no type restrictions  I e  all names of all struct members had independent global meaning  and  therefore  had to be unique   For example you could declare  struct S     int a    int b       and name a would stand for offset 0  while name b would stand for offset 2  assuming int type of size 2 and no padding   The language required all members of all structs in the translation unit either have unique names or stand for the same offset value  E g  in the same translation unit you could additionally declare  struct X     int a    int x       and that would be OK  since the name a would consistently stand for offset 0  But this additional declaration  struct Y     int b    int a       would be formally invalid  since it attempted to  redefine  a as offset 2 and b as offset 0   And this is where the - gt  operator comes in  Since every struct member name had its own self-sufficient global meaning  the language supported expressions like these  int i   5  i- gt b   42      Write 42 into  int  at address 7    100- gt a   0     Write 0 into  int  at address 100      The first assignment was interpreted by the compiler as  take address 5  add offset 2 to it and assign 42 to the int value at the resultant address   I e  the above would assign 42 to int value at address 7  Note that this use of - gt  did not care about the type of the expression on the left-hand side  The left hand side was interpreted as an rvalue numerical address  be it a pointer or an integer    This sort of trickery was not possible with   and   combination  You could not do    i  b   42    since  i is already an invalid expression  The   operator  since it is separate from    imposes more strict type requirements on its operand  To provide a capability to work around this limitation CRM introduced the - gt  operator  which is independent from the type of the left-hand operand   As Keith noted in the comments  this difference between - gt  and     combination is what CRM is referring to as  relaxation of the requirement  in 7 1 8  Except for the relaxation of the requirement that E1 be of pointer type  the expression E1- gt MOS is exactly equivalent to   E1  MOS  Later  in K amp R C many features originally described in CRM were significantly reworked  The idea of  struct member as global offset identifier  was completely removed  And the functionality of - gt  operator became fully identical to the functionality of   and   combination   Why can t   dereference the pointer automatically   Again  in CRM version of the language the left operand of the   operator was required to be an lvalue  That was the only requirement imposed on that operand  and that s what made it different from - gt   as explained above   Note that CRM did not require the left operand of   to have a struct type  It just required it to be an lvalue  any lvalue  This means that in CRM version of C you could write code like this  struct S   int a  b     struct T   float x  y  z      struct T c  c b   55    In this case the compiler would write 55 into an int value positioned at byte-offset 2 in the continuous memory block known as c  even though type struct T had no field named b  The compiler would not care about the actual type of c at all  All it cared about is that c was an lvalue  some sort of writable memory block    Now note that if you did this  S  s      s b   42    the code would be considered valid  since s is also an lvalue  and the compiler would simply attempt to write data into the pointer s itself  at byte-offset 2  Needless to say  things like this could easily result in memory overrun  but the language did not concern itself with such matters   I e  in that version of the language your proposed idea about overloading operator   for pointer types would not work  operator   already had very specific meaning when used with pointers  with lvalue pointers or with any lvalues at all   It was very weird functionality  no doubt  But it was there at the time   Of course  this weird functionality is not a very strong reason against introducing overloaded   operator for pointers  as you suggested  in the reworked version of C - K amp R C  But it hasn t been done  Maybe at that time there was some legacy code written in CRM version of C that had to be supported    The URL for the 1975 C Reference Manual may not be stable  Another copy  possibly with some subtle differences  is here

[c] Why does the arrow (->) operator in C exist?

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