Why use pointers

Question

I know this is a really basic question  but I ve just started with some basic C   programming after coding a few projects with high-level languages    Basically I have three questions    Why use pointers over normal variables  When and where should I use pointers  How do you use pointers with arrays

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Here s my anwser  and I won t promse to be an expert  but I ve found pointers to be great in one of my libraries I m trying to write   In this library  It s a graphics API with OpenGL -   you can create a triangle with vertex objects passed into them   The draw method takes these triangle objects  and well   draws them based on the vertex objects i created   Well  its ok     But  what if i change a vertex coordinate   Move it or something with moveX   in the vertex class   Well  ok  now i have to update the triangle  adding more methods and performance is being wasted because i have to update the triangle every time a vertex moves   Still not a big deal  but it s not that great     Now  what if i have a mesh with tons of vertices and tons of triangles  and the mesh is rotateing  and moveing  and such   I ll have to update every triangle that uses these vertices  and probably every triangle in the scene because i wouldn t know which ones use which vertices   That s hugely computer intensive  and if I have several meshes ontop of a landscape  oh god   I m in trouble  because im updateing every triangle almost every frame because these vertices are changing al the time   With pointers  you don t have to update the triangles   If I had three  Vertex objects per triangle class  not only am i saving room because a zillion triangles don t have three vertex objects which are large themselves  but also these pointers will always point to the Vertices they are meant to  no matter how often the vertices change   Since the pointers still point to the same vertex  the triangles don t change  and the update process is easier to handle   If I confused you  I wouldn t doubt it  I don t pretend to be an expert  just throwing my two cents into the discussion

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In C    if you want to use subtype polymorphism  you have to use pointers  See this post  C   Polymorphism without pointers   Really  when you think about it  this makes sense  When you use subtype polymorphism  ultimately  you don t know ahead of time which class s or subclass s implementation of the method will be invoked because you don t know what the actual class is   This idea of having a variable that holds an object of an unknown class is incompatible with C   s default  non-pointer  mode of storing objects on the stack  where the amount of space allocated directly corresponds to the class   Note  if a class has 5 instance fields versus 3  more space will need to be allocated    Note that if you are using   amp   to pass arguments by reference  indirection  i e   pointers  is still involved behind the scenes  The   amp   is just syntactic sugar that  1  saves you the trouble of using pointer syntax and  2  allows the compiler to be more strict  such as prohibiting null pointers

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One reason to use pointers is so that a variable or an object can be modified in a called function   In C   it is a better practice to use references than pointers  Though references are essentially pointers  C   to some extent hides the fact and makes it seem as if you are passing by value  This makes it easy to change the way the calling function receives the value without having to modify the semantics of passing it   Consider the following examples   Using references   public void doSomething         int i   10      doSomethingElse i       passes i by references since doSomethingElse   receives it                             by reference  but the syntax makes it appear as if i is passed                             by value    public void doSomethingElse int amp  i      receives i as a reference       cout  lt  lt  i  lt  lt  endl      Using pointers   public void doSomething         int i   10      doSomethingElse  amp i      public void doSomethingElse int  i        cout  lt  lt   i  lt  lt  endl

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In some cases  function pointers are required to use functions that are in a shared library   DLL or  so   This includes performing stuff across languages  where oftentimes a DLL interface is provided  Making compilers Making scientific calculators  where you have an array or vector or string map of function pointers  Trying to modify video memory directly - making your own graphics package Making an API  Data structures - node link pointers for special trees you are making   There are Lots of reasons for pointers   Having C name mangling especially is important in DLLs if you want to maintain cross-language compatibility

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Regarding your second question  generally you don t need to use pointers while programming  however there is one exception to this and that is when you make a public API   The problem with C   constructs that people generally use to replace pointers are very dependent on the toolset that you use which is fine when you have all the control you need over the source code  however if you compile a static library with visual studio 2008 for instance and try to use it in a visual studio 2010 you will get a ton of linker errors because the new project is linked with a newer version of STL which is not backwards compatible  Things get even nastier if you compile a DLL and give an import library that people use in a different toolset because in that case your program will crash sooner or later for no apparent reason    So for the purpose of moving large data sets from one library to another you could consider giving a pointer to an array to the function that is supposed to copy the data if you don t want to force others to use the same tools that you use  The good part about this is that it doesn t even have to be a C-style array  you can use a std  vector and give the pointer by giving the address of the first element  amp vector 0  for instance  and use the std  vector to manage the array internally    Another good reason to use pointers in C   again relates to libraries  consider having a dll that cannot be loaded when your program runs  so if you use an import library then the dependency isn t satisfied and the program crashes  This is the case for instance when you give a public api in a dll alongside your application and you want to access it from other applications  In this case in order to use the API you need to load the dll from its  location  usually it s in a registry key  and then you need to use a function pointer to be able to call functions inside the DLL  Sometimes the people that make the API are nice enough to give you a  h file that contain helper functions to automate this process and give you all the function pointers that you need  but if not you can use LoadLibrary and GetProcAddress on windows and dlopen and dlsym on unix to get them  considering that you know the entire signature of the function

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Pointers are one way of getting an indirect reference to another variable   Instead of holding the value of a variable  they tell you its address   This is particularly useful when dealing with arrays  since using a pointer to the first element in an array  its address  you can quickly find the next element by incrementing the pointer  to the next address location    The best explanation of pointers and pointer arithmetic that I ve read is in K  amp  R s The C Programming Language   A good book for beginning learning C   is C   Primer

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Because copying big objects all over the places wastes time and memory

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Here s a slightly different  but insightful take on why many features of C make sense  http   steve yegge googlepages com tour-de-babel C  Basically  the standard CPU architecture is a Von Neumann architecture  and it s tremendously useful to be able to refer to the location of a data item in memory  and do arithmetic with it  on such a machine   If you know any variant of assembly language  you will quickly see how crucial this is at the low level   C   makes pointers a bit confusing  since it sometimes manages them for you and hides their effect in the form of  references    If you use straight C  the need for pointers is much more obvious  there s no other way to do call-by-reference  it s the best way to store a string  it s the best way to iterate through an array  etc

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Let me try and answer this too    Pointers are similar to references  In other words  they re not copies  but rather a way to refer to the original value    Before anything else  one place where you will typically have to use pointers a lot is when you re dealing with embedded hardware  Maybe you need to toggle the state of a digital IO pin  Maybe you re processing an interrupt and need to store a value at a specific location  You get the picture  However  if you re not dealing with hardware directly and are just wondering about which types to use  read on   Why use pointers as opposed to normal variables  The answer becomes clearer when you re dealing with complex types  like classes  structures and arrays  If you were to use a normal variable  you might end up making a copy  compilers are smart enough to prevent this in some situations and C  11 helps too  but we ll stay away from that discussion for now     Now what happens if you want to modify the original value  You could use something like this   MyType a    let s ignore what MyType actually is right now  a   modify a      That will work just fine and if you don t know exactly why you re using pointers  you shouldn t use them  Beware of the  they re probably faster  reason  Run your own tests and if they actually are faster  then use them   However  let s say you re solving a problem where you need to allocate memory  When you allocate memory  you need to deallocate it  The memory allocation may or may not be successful  This is where pointers come in useful - they allow you to test for the existence of the object you ve allocated and they allow you to access the object the memory was allocated for by de-referencing the pointer   MyType  p   NULL    empty pointer if p          we never reach here  because the pointer points to nothing     now  let s allocate some memory p   new MyType 50000   if p    if the memory was allocated  this test will pass         we can do something with our allocated array     for size t i 0  i  50000  i                  MyType  amp v     p i     get a reference to the ith object           do something with it                         delete   p    we re done  de-allocate the memory     This is the key to why you would use pointers - references assume the element you re referencing exists already  A pointer does not    The other reason why you would use pointers  or at least end up having to deal with them  is because they re a data type that existed before references  Therefore  if you end up using libraries to do the things that you know they re better at  you will find that a lot of these libraries use pointers all over the place  simply because of how long they ve been around  a lot of them were written before C       If you didn t use any libraries  you could design your code in such a way that you could stay away from pointers  but given that pointers are one of the basic types of the language  the faster you get comfortable using them  the more portable your C   skills would be   From a maintainability point of view  I should also mention that when you do use pointers  you either have to test for their validity and handle the case when they re not valid  or  just assume they are valid and accept the fact that your program will crash or worse WHEN that assumption is broken  Put another way  your choice with pointers is to either introduce code complexity or more maintenance effort when something breaks and you re trying to track down a bug that belongs to a whole class of errors that pointers introduce  like memory corruption    So if you control all of your code  stay away from pointers and instead use references  keeping them const when you can  This will force you to think about the life times of your objects and will end up keeping your code easier to understand    Just remember this difference  A reference is essentially a valid pointer  A pointer is not always valid   So am I saying that its impossible to create an invalid reference  No  Its totally possible  because C   lets you do almost anything  It s just harder to do unintentionally and you will be amazed at how many bugs are unintentional

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Why use pointers over normal variables     Short answer is  Don t   -  Pointers are to be used where you can t use anything else  It is either because the lack of appropriate functionality  missing data types or for pure perfomance  More below      When and where should I use pointers     Short answer here is  Where you cannot use anything else  In C you don t have any support for complex datatypes such as a string  There are also no way of passing a variable  by reference  to a function  That s where you have to use pointers  Also you can have them to point at virtually anything  linked lists  members of structs and so on  But let s not go into that here    How do you use pointers with arrays     With little effort and much confusion   -  If we talk about simple data types such as int and char there is little difference between an array and a pointer  These declarations are very similar  but not the same - e g   sizeof will return different values    char  a    Hello   char a      Hello     You can reach any element in the array like this  printf  Second char is   c   a 1      Index 1 since the array starts with element 0   -   Or you could equally do this  printf  Second char is   c     a 1      The pointer operator  the    is needed since we are telling printf that we want to print a character  Without the    the character representation of the memory address itself would be printed  Now we are using the character itself instead  If we had used  s instead of  c  we would have asked printf to print the content of the memory address pointed to by  a  plus one  in this example above   and we wouldn t have had to put the   in front   printf  Second char is   s    a 1       WRONG      But this would not have just printed the second character  but instead all characters in the next memory addresses  until a null character   0  were found  And this is where things start to get dangerous  What if you accidentally try and print a variable of the type integer instead of a char pointer with the  s formatter   char  a    Hello   int b   120  printf  Second char is   s   b     This would print whatever is found on memory address 120 and go on printing until a null character was found  It is wrong and illegal to perform this printf statement  but it would probably work anyway  since a pointer actually is of the type int in many environments  Imagine the problems you might cause if you were to use sprintf   instead and assign this way too long  char array  to another variable  that only got a certain limited space allocated  You would most likely end up writing over something else in the memory and cause your program to crash  if you are lucky    Oh  and if you don t assign a string value to the char array   pointer when you declare it  you MUST allocate sufficient amount of memory to it before giving it a value  Using malloc  calloc or similar  This since you only declared one element in your array   one single memory address to point at  So here s a few examples   char  x     Allocate 6 bytes of memory for me and point x to the first of them     x    char   malloc 6   x 0     H   x 1     e   x 2     l   x 3     l   x 4     o   x 5      0   printf  String    s   at address   d n   x  x      Delete the allocation  reservation  of the memory        The char pointer x is still pointing to this address in memory though     free x      Same as malloc but here the allocated space is filled with null characters    x    char    calloc 6  sizeof x    x 0     H   x 1     e   x 2     l   x 3     l   x 4     o   x 5      0   printf  String    s   at address   d n   x  x      And delete the allocation again       free x      We can set the size at declaration time as well    char xx 6   xx 0     H   xx 1     e   xx 2     l   xx 3     l   xx 4     o   xx 5      0   printf  String    s   at address   d n   xx  xx     Do note that you can still use the variable x after you have performed a free   of the allocated memory  but you do not know what is in there  Also do notice that the two printf   might give you different addresses  since there is no guarantee that the second allocation of memory is performed in the same space as the first one

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In java and C  all the object references are pointers  the thing with c   is that you have more control on where you pointer points  Remember With great power comes grand responsibility

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One way to use pointers over variables is to eliminate duplicate memory required   For example  if you have some large complex object  you can use a pointer to point to that variable for each reference you make   With a variable  you need to duplicate the memory for each copy

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The need for pointers in C language is described here The basic idea is that many limitations in the language  like using arrays  strings and modifying multiple variables in functions  could be removed by manipulating with the memory location of the data  To overcome these limitations  pointers were introduced in C  Further  it is also seen that using pointers  you can run your code faster and save memory in cases where you are passing big data types  like a structure with many fields  to a function  Making a copy of such data types before passing would take time and would consume memory  This is another reason why programmers prefer pointers for big data types  PS  Please refer the link provided for detailed explanation with sample code

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Pointers allow you to refer to the same space in memory from multiple locations  This means that you can update memory in one location and the change can be seen from another location in your program  You will also save space by being able to share components in your data structures  You should use pointers any place where you need to obtain and pass around the address to a specific spot in memory  You can also use pointers to navigate arrays  An array is a block of contiguous memory that has been allocated with a specific type  The name of the array contains the value of the starting spot of the array  When you add 1  that takes you to the second spot  This allows you to write loops that increment a pointer that slides down the array without having an explicit counter for use in accessing the array    Here is an example in C   char hello      hello    char  p   hello   while   p         p    1     increase the character by one      p    1     move to the next spot    printf hello     prints  ifmmp   because it takes the value for each character and increments it by one

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In large part  pointers are arrays  in C C    - they are addresses in memory  and can be accessed like an array if desired  in  normal  cases    Since they re the address of an item  they re small  they take up only the space of an address  Since they re small  sending them to a function is cheap  And then they allow that function to work on the actual item rather than a copy   If you want to do dynamic storage allocation  such as for a linked-list   you must use pointers  because they re the only way to grab memory from the heap

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Pointers are important in many data structures whose design requires the ability to link or chain one  node  to another efficiently   You would not  choose  a pointer over say a normal data type like float  they simply have different purposes   Pointers are useful where you require high performance and or compact memory footprint   The address of the first element in your array can be assigned to a pointer   This then allows you to access the underlying allocated bytes directly   The whole point of an array is to avoid you needing to do this though

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One use of pointers  I won t mention things already covered in other people s posts  is to access memory that you haven t allocated   This isn t useful much for PC programming  but it s used in embedded programming to access memory mapped hardware devices   Back in the old days of DOS  you used to be able to access the video card s video memory directly by declaring a pointer to   unsigned char  pVideoMemory    unsigned char   0xA0000000    Many embedded devices still use this technique

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