What does dereferencing a pointer mean

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Please include an example with the explanation

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In simple words  dereferencing means accessing the value from a certain memory location against which that pointer is pointing

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Code and explanation from Pointer Basics      The dereference operation starts at   the pointer and follows its arrow over   to access its pointee  The goal may be   to look at the pointee state or to   change the pointee state  The   dereference operation on a pointer   only works if the pointer has a   pointee -- the pointee must be   allocated and the pointer must be set   to point to it  The most common error   in pointer code is forgetting to set   up the pointee  The most common   runtime crash because of that error in   the code is a failed dereference   operation  In Java the incorrect   dereference will be flagged politely   by the runtime system  In compiled   languages such as C  C    and Pascal    the incorrect dereference will   sometimes crash  and other times   corrupt memory in some subtle  random   way  Pointer bugs in compiled   languages can be difficult to track   down for this reason    void main            int     x      Allocate the pointer x     x   malloc sizeof int          Allocate an int pointee                                 and set x to point to it      x   42        Dereference x to store 42 in its pointee

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A pointer is a  reference  to a value   much like a library call number is a reference to a book   Dereferencing  the call number is physically going through and retrieving that book   int a 4   int  pA    amp a   printf   The REFERENCE call number for the variable  a  is  p n   pA         The   causes pA to DEREFERENCE      a  via  callnumber   pA   printf    d n    pA        prints 4      If the book isn t there  the librarian starts shouting  shuts the library down  and a couple of people are set to investigate the cause of a person going to find a book that isn t there

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Reviewing the basic terminology  It s usually good enough - unless you re programming assembly - to envisage a pointer containing a numeric memory address  with 1 referring to the second byte in the process s memory  2 the third  3 the fourth and so on       What happened to 0 and the first byte   Well  we ll get to that later - see null pointers below  For a more accurate definition of what pointers store  and how memory and addresses relate  see  More about memory addresses  and why you probably don t need to know  at the end of this answer    When you want to access the data value in the memory that the pointer points to - the contents of the address with that numerical index - then you dereference the pointer   Different computer languages have different notations to tell the compiler or interpreter that you re now interested in the pointed-to object s  current  value - I focus below on C and C     A pointer scenario  Consider in C  given a pointer such as p below     const char  p    abc        four bytes with the numerical values used to encode the letters  a    b    c   and a 0 byte to denote the end of the textual data  are stored somewhere in memory and the numerical address of that data is stored in p   This way C encodes text in memory is known as ASCIIZ    For example  if the string literal happened to be at address 0x1000 and p a 32-bit pointer at 0x2000  the memory content would be   Memory Address  hex     Variable name    Contents 1000                                      a     97  ASCII  1001                                      b     98 1002                                      c     99 1003                                     0     2000-2003               p                1000 hex   Note that there is no variable name identifier for address 0x1000  but we can indirectly refer to the string literal using a pointer storing its address  p   Dereferencing the pointer  To refer to the characters p points to  we dereference p using one of these notations  again  for C    assert  p     a        The first character at address p will be  a  assert p 1      b       p 1  actually dereferences a pointer created by adding                         p and 1 times the size of the things to which p points                          In this case they re char which are 1 byte in C    assert   p   1      b        Another notation for p 1    You can also move pointers through the pointed-to data  dereferencing them as you go     p      Increment p so it s now 0x1001 assert  p     b        p    0x1001 which is where the  b  is      If you have some data that can be written to  then you can do things like this   int x   2  int  p x    amp x      Put the address of the x variable into the pointer p x  p x   4           Change the memory at the address in p x to be 4 assert x    4      Check x is now 4   Above  you must have known at compile time that you would need a variable called x  and the code asks the compiler to arrange where it should be stored  ensuring the address will be available via  amp x   Dereferencing and accessing a structure data member  In C  if you have a variable that is a pointer to a structure with data members  you can access those members using the - gt  dereferencing operator   typedef struct X   int i   double d     X  X x  X  p    amp x  p- gt d    3 14159      Dereference and access data member x d    p  d     -1        Another equivalent notation for accessing x d    Multi-byte data types  To use a pointer  a computer program also needs some insight into the type of data that is being pointed at - if that data type needs more than one byte to represent  then the pointer normally points to the lowest-numbered byte in the data   So  looking at a slightly more complex example   double sizes       10 3  13 4  11 2  19 4    double  p   sizes  assert p 0     10 3       Knows to look at all the bytes in the first double value assert p 1     13 4       Actually looks at bytes from address p   1   sizeof double                             sizeof double  is almost always eight bytes    p                       Advance p by sizeof double  assert  p    13 4         The double at memory beginning at address p has value 13 4   p   2    29 8           Change sizes 3  from 19 4 to 29 8                           Note earlier   p and   2 here   gt  sizes 3    Pointers to dynamically allocated memory  Sometimes you don t know how much memory you ll need until your program is running and sees what data is thrown at it    then you can dynamically allocate memory using malloc  It is common practice to store the address in a pointer     int  p    int  malloc sizeof int       Get some memory somewhere     p   10                Dereference the pointer to the memory  then write a value in fn  p                  Call a function  passing it the value at address p   p     3              Change the value  adding 3 to it free p                 Release the memory back to the heap allocation library   In C    memory allocation is normally done with the new operator  and deallocation with delete   int  p   new int 10      Memory for one int with initial value 10 delete p   p   new int 10           Memory for ten ints with unspecified initial value delete   p   p   new int 10           Memory for ten ints that are value initialised  to 0  delete   p    See also C   smart pointers below   Losing and leaking addresses  Often a pointer may be the only indication of where some data or buffer exists in memory  If ongoing use of that data buffer is needed  or the ability to call free   or delete to avoid leaking the memory  then the programmer must operate on a copy of the pointer     const char  p   asprintf  name   s   name       Common but non-Standard printf-on-heap     Replace non-printable characters with underscores     for  const char  q   p   q    q      if   isprint  q            q         printf   s n   p      Only q was modified free p        or carefully orchestrate reversal of any changes     const size t n        p    n      p -  n      Restore earlier value    free p     C   smart pointers  In C    it s best practice to use smart pointer objects to store and manage the pointers  automatically deallocating them when the smart pointers  destructors run  Since C  11 the Standard Library provides two  unique ptr for when there s a single owner for an allocated object           std  unique ptr lt T gt  p new T 42   meaning         call a function p          The function above might throw  so delete here is unreliable  but         p s destructor s guaranteed to run  here   calling delete      and shared ptr for share ownership  using reference counting            auto p   std  make shared lt T gt  3 14   pi        number storage1 may add p      Might copy p into its container     number storage2 may add p      Might copy p into its container         p s destructor will only delete the T if neither may add copied it   Null pointers  In C  NULL and 0 - and additionally in C   nullptr - can be used to indicate that a pointer doesn t currently hold the memory address of a variable  and shouldn t be dereferenced or used in pointer arithmetic  For example   const char  p filename   NULL     Or    0   or    nullptr  in C   int c  while   c   getopt argc  argv   f        -1      switch  c          case f  p filename   optarg  break        if  p filename      Only NULL converts to false              Only get here if -f flag specified   In C and C    just as inbuilt numeric types don t necessarily default to 0  nor bools to false  pointers are not always set to NULL  All these are set to 0 false NULL when they re static variables or  C   only  direct or indirect member variables of static objects or their bases  or undergo zero initialisation  e g  new T    and new T x  y  z   perform zero-initialisation on T s members including pointers  whereas new T  does not    Further  when you assign 0  NULL and nullptr to a pointer the bits in the pointer are not necessarily all reset  the pointer may not contain  0  at the hardware level  or refer to address 0 in your virtual address space  The compiler is allowed to store something else there if it has reason to  but whatever it does - if you come along and compare the pointer to 0  NULL  nullptr or another pointer that was assigned any of those  the comparison must work as expected  So  below the source code at the compiler level   NULL  is potentially a bit  magical  in the C and C   languages     More about memory addresses  and why you probably don t need to know  More strictly  initialised pointers store a bit-pattern identifying either NULL or a  often virtual  memory address   The simple case is where this is a numeric offset into the process s entire virtual address space  in more complex cases the pointer may be relative to some specific memory area  which the CPU may select based on CPU  segment  registers or some manner of segment id encoded in the bit-pattern  and or looking in different places depending on the machine code instructions using the address   For example  an int  properly initialised to point to an int variable might - after casting to a float  - access memory in  GPU  memory quite distinct from the memory where the int variable is  then once cast to and used as a function pointer it might point into further distinct memory holding machine opcodes for the program  with the numeric value of the int  effectively a random  invalid pointer within these other memory regions    3GL programming languages like C and C   tend to hide this complexity  such that    If the compiler gives you a pointer to a variable or function  you can dereference it freely  as long as the variable s not destructed deallocated meanwhile  and it s the compiler s problem whether e g  a particular CPU segment register needs to be restored beforehand  or a distinct machine code instruction used If you get a pointer to an element in an array  you can use pointer arithmetic to move anywhere else in the array  or even to form an address one-past-the-end of the array that s legal to compare with other pointers to elements in the array  or that have similarly been moved by pointer arithmetic to the same one-past-the-end value   again in C and C    it s up to the compiler to ensure this  just works  Specific OS functions  e g  shared memory mapping  may give you pointers  and they ll  just work  within the range of addresses that makes sense for them Attempts to move legal pointers beyond these boundaries  or to cast arbitrary numbers to pointers  or use pointers cast to unrelated types  typically have undefined behaviour  so should be avoided in higher level libraries and applications  but code for OSes  device drivers  etc  may need to rely on behaviour left undefined by the C or C   Standard  that is nevertheless well defined by their specific implementation or hardware

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Dereferencing a pointer means getting the value that is stored in the memory location pointed by the pointer  The operator   is used to do this  and is called the dereferencing operator   int a   10  int  ptr    amp a   printf   d    ptr      With  ptr I m dereferencing the pointer                          Which means  I am asking the value pointed at by the pointer                         ptr is pointing to the location in memory of the variable a                         In a s location  we have 10  So  dereferencing gives this value      Since we have indirect control over a s location  we can modify its content using the pointer  This is an indirect way to access a     ptr   20             Now a s content is no longer 10  and has been modified to 20

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I think all the previous answers are wrong  as they state that dereferencing means accessing the actual value  Wikipedia gives the correct definition instead  https   en wikipedia org wiki Dereference operator     It operates on a pointer variable  and returns an l-value equivalent to the value at the pointer address  This is called  dereferencing  the pointer    That said  we can dereference the pointer without ever accessing the value it points to  For example   char  p   NULL   p    We dereferenced the NULL pointer without accessing its value  Or we could do   p1    amp   p   sz   sizeof  p     Again  dereferencing  but never accessing the value  Such code will NOT crash  The crash happens when you actually access the data by an invalid pointer  However  unfortunately  according the the standard  dereferencing an invalid pointer is an undefined behaviour  with a few exceptions   even if you don t try to touch the actual data   So in short  dereferencing the pointer means applying the dereference operator to it  That operator just returns an l-value for your future use

[c++] What does "dereferencing" a pointer mean?

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