How to compare pointers

Question

Suppose I have 2 pointers   int  a   something  int  b   something    If I want to compare them and see if they point at the same place does  a    b  work

User · Accepted Answer

Yes  that is the definition of raw pointer equality  they both point to the same location  or are pointer aliases   usually in the virtual address space of the process running your application coded in C   and managed by some operating system  but C   can also be used for programming embedded devices with micro-controllers having a Harward architecture  on such microcontrollers some pointer casts are forbidden and makes no sense - since read only data could sit in code ROM  For C    read a good C   programming book  see this C   reference website  read the documentation of your C   compiler  perhaps GCC or Clang  and consider coding with smart pointers  Maybe read also some draft C   standard  like n4713 or buy the official standard from your ISO representative  The concepts and terminology of garbage collection are also relevant when managing pointers and memory zones obtained by dynamic allocation  e g    operator new   so read perhaps the GC handbook  For pointers on Linux machines  see also this

User · Answer

Comparing pointers is not portable  for example in DOS different pointer values points to the same location  comparison of the pointers returns false    --    main c --------------------------------------------------    include  lt dos h gt   include  lt stdio h gt   include  lt stdlib h gt  int main void      int   val a   123    int   ptr 0    amp val a    int   ptr 1   MK FP FP SEG  amp val a    1  FP OFF  amp val a  - 16      printf  quot  val a    d - gt    p n quot   val a   void     amp val a      printf  quot  ptr 0    d - gt    p n quot    ptr 0   void   ptr 0     printf  quot  ptr 1    d - gt    p n quot    ptr 1   void   ptr 1         Check what returns the pointers comparison       printf  quot  amp val a    ptr 0      gt   d n quot    amp val a    ptr 0     printf  quot  amp val a    ptr 1      gt   d n quot    amp val a    ptr 1     printf  quot  ptr 0    ptr 1      gt   d n quot    ptr 0    ptr 1      printf  quot val a    d n quot   val a      printf  quot  gt  gt   ptr 0    100  n quot                  ptr 0    100     printf  quot val a    d n quot   val a      printf  quot  gt  gt   ptr 1    500  n quot                  ptr 1    500     printf  quot val a    d n quot   val a      return EXIT SUCCESS      -- -- main c --------------------------------------------------    Compile it under Borland C 5 0  here is the result    --    result --------------------------------------------------    val a   123 - gt   167A 0FFE  ptr 0   123 - gt   167A 0FFE  ptr 1   123 - gt   167B 0FEE  amp val a    ptr 0      gt  1  amp val a    ptr 1      gt  0  ptr 0    ptr 1      gt  0 val a   123  gt  gt   ptr 0    100  val a   223  gt  gt   ptr 1    500  val a   723   -- -- result --------------------------------------------------

User · Answer

The    operator on pointers will compare their numeric address and hence determine if they point to the same object

User · Answer

Simple code to check pointer aliasing   int main          int a   10  b   20      int  p1   p2   p3   p4       p1    amp a      p2    amp a      if p1    p2           std  cout lt  lt  p1 and p2 alias each other  lt  lt std  endl            else          std  cout lt  lt  p1 and p2 do not alias each other  lt  lt std  endl              ------------------------     p3    amp a      p4    amp b      if p3    p4           std  cout lt  lt  p3 and p4 alias each other  lt  lt std  endl            else          std  cout lt  lt  p3 and p4 do not alias each other  lt  lt std  endl            return 0      Output   p1 and p2 alias each other p3 and p4 do not alias each other

User · Answer

For a bit of facts here is the relevant text from the specifications  Equality operator          Pointers to objects of the same type can be compared for equality with the  intuitive  expected results   From    5 10  of the C  11 standard      Pointers of the same type    after pointer conversions  can be compared for equality  Two pointers of the same type compare equal if   and only if they are both null  both point to the same function  or both represent the same address  3 9 2         leaving out details on comparison of pointers to member and or the null pointer constants - they continue down the same line of  Do What I Mean                  If both operands are null  they compare equal  Otherwise if only one is null  they compare unequal                The most  conspicuous  caveat has to do with virtuals  and it does seem to be the logical thing to expect too                if either is a pointer to a virtual member function  the result is unspecified  Otherwise they   compare equal if and only if they would refer to the same member of the same most derived object  1 8    or the same subobject if they were dereferenced with a hypothetical object of the associated class type              Relational operators   lt     lt        From    5 9 of the C  11 standard      Pointers to objects or functions of the same type  after pointer conversions  can be compared    with a result defined as follows          If two pointers p and q of the same type point to the same object or   function  or both point one past the end of the same array  or are both   null  then p lt  q and p gt  q both yield true and p lt q and p gt q both yield false    If two pointers p and q of the same type point to different objects that are   not members of the same object or elements of the same array or to different   functions  or if only one of them is null  the results of p lt q  p gt q  p lt  q  and   p gt  q are unspecified    If two pointers point to non-static data members of the same object  or to   subobjects or array elements of such members  recursively  the pointer to the   later declared member compares greater provided the two members have the same   access control  Clause 11  and provided their class is not a union    If two pointers point to non-static data members of the same object with   different access control  Clause 11  the result is unspecified    If two pointers point to non-static data members of the same union object    they compare equal  after conversion to void   if necessary   If two pointers   point to elements of the same array or one beyond the end of the array  the   pointer to the object with the higher subscript compares higher    Other pointer comparisons are unspecified       So  if you had   int arr 3   int  a   arr  int  b   a   1  assert a    b      OK  well defined   Also OK   struct X   int x y    s  int  a    amp s x  int  b    amp s y  assert b  gt  a      OK  well defined   But it depends on the something in your question   int g   int main          int h       int i        int  a    amp g       int  b    amp h     can t compare a  lt   gt  b      int  c    amp i     can t compare b  lt   gt  c  or a  lt   gt  c etc          but a  b  b  c  a  c etc  are supported just fine     Bonus  what else is there in the standard library      20 8 5 8   For templates greater  less  greater equal  and less equal  the specializations for any pointer type  yield a total order  even if the built-in operators  lt    gt    lt     gt   do not    So  you can globally order any odd void  as long as you use std  less lt  gt  and friends  not bare operator lt

User · Answer

To sum up  If we want to see if two pointers point to the same memory location we can do that  Also if we want to compare the contents of the memory pointed to by two pointers we can do that too  just remeber to dereference them first   If we have   int  a   something   int  b   something    which are two pointers of the same type we can   Compare memory address   a  b   and compare contents    a   b

User · Answer

It depends on the types of the values  and the way that operators happen to have been defined   For example  string comparison is by value  not by address   But char   is by address normally  I think   A big trap for the unwary   There is no guaranteed pointer comparison operator  but    void   a     void   b   is probably fairly safe

[c++] How to compare pointers?

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