What exactly is nullptr

Question

We now have C  11 with many new features  An interesting and confusing one  at least for me  is the new nullptr   Well  no need anymore for the nasty macro NULL   int  x   nullptr  myclass  obj   nullptr    Still  I am not getting how nullptr works  For example  Wikipedia article says      C  11 corrects this by introducing a new keyword to serve as a distinguished null pointer constant  nullptr  It is of type nullptr t  which is implicitly convertible and comparable to any pointer type or pointer-to-member type  It is not implicitly convertible or comparable to integral types  except for bool    How is it a keyword and an instance of a type   Also  do you have another example  beside the Wikipedia one  where nullptr is superior to good old 0

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When you have a function that can receive pointers to more than one type, calling it with NULL is ambiguous. The way this is worked around now is very hacky by accepting an int and assuming it's NULL.

template <class T>
class ptr {
    T* p_;
    public:
        ptr(T* p) : p_(p) {}

        template <class U>
        ptr(U* u) : p_(dynamic_cast<T*>(u)) { }

        // Without this ptr<T> p(NULL) would be ambiguous
        ptr(int null) : p_(NULL)  { assert(null == NULL); }
};

In C++11 you would be able to overload on nullptr_t so that ptr<T> p(42); would be a compile-time error rather than a run-time assert.

ptr(std::nullptr_t) : p_(nullptr)  {  }

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Why nullptr in C  11  What is it  Why is NULL not sufficient   C   expert Alex Allain says it perfectly here  my emphasis added in bold          imagine you have the following two function declarations   void func int n    void func char  s    func  NULL       guess which function gets called        Although it looks like the second function will be called--you are  after all  passing in what seems to be a pointer--it s really the first function that will be called  The trouble is that because NULL is 0  and 0 is an integer  the first version of func will be called instead  This is the kind of thing that  yes  doesn t happen all the time  but when it does happen  is extremely frustrating and confusing  If you didn t know the details of what is going on  it might well look like a compiler bug  A language feature that looks like a compiler bug is  well  not something you want       Enter nullptr  In C  11  nullptr is a new keyword that can  and should   be used to represent NULL pointers  in other words  wherever you were writing NULL before  you should use nullptr instead  It s no more clear to you  the programmer   everyone knows what NULL means   but it s more explicit to the compiler  which will no longer see 0s everywhere being used to have special meaning when used as a pointer    Allain ends his article with      Regardless of all this--the rule of thumb for C  11 is simply to start using nullptr whenever you would have otherwise used NULL in the past     My words    Lastly  don t forget that nullptr is an object--a class  It can be used anywhere NULL was used before  but if you need its type for some reason  it s type can be extracted with decltype nullptr   or directly described as std  nullptr t  which is simply a typedef of decltype nullptr    References    Cprogramming com  Better types in C  11 - nullptr  enum classes  strongly typed enumerations  and cstdint https   en cppreference com w cpp language decltype https   en cppreference com w cpp types nullptr t

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Also  do you have another example  beside the Wikipedia one  where nullptr is superior to good old 0    Yes  It s also a  simplified  real-world example that occurred in our production code  It only stood out because gcc was able to issue a warning when crosscompiling to a platform with different register width  still not sure exactly why only when crosscompiling from x86 64 to x86  warns warning  converting to non-pointer type  int  from NULL    Consider this code  C  03     include  lt iostream gt   struct B      struct A       operator B     return 0       operator bool    return true       int main         A a      B  pb   0      typedef void  null ptr t      null ptr t null   0       std  cout  lt  lt    a    pb      lt  lt   a    pb   lt  lt  std  endl      std  cout  lt  lt    a    0      lt  lt   a    0   lt  lt  std  endl     no warning     std  cout  lt  lt    a    NULL      lt  lt   a    NULL   lt  lt  std  endl     warns sometimes     std  cout  lt  lt    a    null      lt  lt   a    null   lt  lt  std  endl      It yields this output    a    pb   1  a    0   0  a    NULL   0  a    null   1

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0 used to be the only integer value that could be used as a cast-free initializer for pointers  you can not initialize pointers with other integer values without a cast  You can consider 0 as a consexpr singleton syntactically similar to an integer literal  It can initiate any pointer or integer  But surprisingly  you ll find that it has no distinct type  it is an int  So how come 0 can initialize pointers and 1 cannot  A practical answer was we need a means of defining pointer null value and direct implicit conversion of int to a pointer is error-prone  Thus 0 became a real freak weirdo beast out of the prehistoric era  nullptr was proposed to be a real singleton constexpr representation of null value to initialize pointers  It can not be used to directly initialize integers and eliminates ambiguities involved with defining NULL in terms of 0  nullptr could be defined as a library using std syntax but semantically looked to be a missing core component  NULL is now deprecated in favor of nullptr  unless some library decides to define it as nullptr

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NULL need not to be 0  As long you use always NULL and never 0  NULL can be any value  Asuming you programme a von Neuman Microcontroller with flat memory  that has its interrupt vektors at 0  If NULL is 0 and something writes at a NULL Pointer the Microcontroller crashes  If NULL is lets say 1024 and at 1024 there is a reserved variable  the write won t crash it  and you can detect NULL Pointer assignments from inside the programme  This is Pointless on PCs  but for space probes  military or medical equipment it is important not to crash

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According to cppreference  nullptr is a keyword that   denotes the pointer literal  It is a prvalue of type std  nullptr t  There exist implicit conversions from nullptr to null pointer value of any pointer type and any pointer to member type  Similar conversions exist for any null pointer constant  which includes values of type std  nullptr t as well as the macro NULL   So nullptr is a value of a distinct type std  nullptr t  not int  It implicitly converts to the null pointer value of any pointer type  This magic happens under the hood for you and you don t have to worry about its implementation   NULL  however  is a macro and it is an implementation-defined null pointer constant  It s often defined like this   define NULL 0  i e  an integer  This is a subtle but important difference  which can avoid ambiguity  For example  int i   NULL        OK int i   nullptr     error int  p   NULL       OK int  p   nullptr    OK  and when you have two function overloads like this  void func int x       1  void func int  x      2   func NULL  calls 1  because NULL is an integer  func nullptr  calls 2  because nullptr converts implicitly to a pointer of type int   Also if you see a statement like this  auto result   findRecord     arguments        if  result    nullptr            and you can t easily find out what findRecord returns  you can be sure that result must be a pointer type  nullptr makes this more readable  In a deduced context  things work a little differently  If you have a template function like this  template lt typename T gt  void func T  ptr               and you try to call it with nullptr  func nullptr    you will get a compiler error because nullptr is of type nullptr t  You would have to either explicitly cast nullptr to a specific pointer type or provide an overload specialization for func with nullptr t   Advantages of using nulptr   avoid ambiguity between function overloads enables you to do template specialization more secure  intuitive and expressive code  e g  if  ptr    nullptr  instead of if  ptr    0

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nullptr can t be assigned to an integral type such as an int but only a pointer type  either a built-in pointer type such as int  ptr or a smart pointer such as std  shared ptr lt T gt   I believe this is an important distinction because NULL can still be assigned to both an integral type and a pointer as NULL is a macro expanded to 0 which can serve as both an initial value for an int as well as a pointer

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Let s say that you have a function  f  which is overloaded to take both int and char   Before C   11  If you wanted to call it with a null pointer  and you used NULL  i e  the value 0   then you would call the one overloaded for int   void f int   void f char     void g        f 0      Calls f int     f NULL      Equals to f 0   Calls f int       This is probably not what you wanted  C  11 solves this with nullptr  Now you can write the following   void g       f nullptr     calls f char

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Here s the LLVM header      - - C   - -      ---------------------------   nullptr --------------------------------            Part of the LLVM Project  under the Apache License v2 0 with LLVM Exceptions     See https   llvm org LICENSE txt for license information     SPDX-License-Identifier  Apache-2 0 WITH LLVM-exception         ----------------------------------------------------------------------        ifndef  LIBCPP NULLPTR  define  LIBCPP NULLPTR   include  lt   config gt    if  defined  LIBCPP HAS NO PRAGMA SYSTEM HEADER   pragma GCC system header  endif   ifdef  LIBCPP HAS NO NULLPTR   LIBCPP BEGIN NAMESPACE STD  struct  LIBCPP TEMPLATE VIS nullptr t       void    lx       struct   nat  int   for bool           LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR nullptr t       lx 0          LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR nullptr t int   nat         lx 0           LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR operator int   nat      const  return 0        template  lt class  Tp gt           LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR         operator  Tp     const  return 0        template  lt class  Tp  class  Up gt           LIBCPP INLINE VISIBILITY         operator  Tp  Up       const  return 0        friend  LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR bool operator   nullptr t  nullptr t   return true       friend  LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR bool operator   nullptr t  nullptr t   return false       inline  LIBCPP INLINE VISIBILITY  LIBCPP CONSTEXPR nullptr t   get nullptr t    return nullptr t 0      define nullptr  VSTD    get nullptr t     LIBCPP END NAMESPACE STD   else      LIBCPP HAS NO NULLPTR  namespace std       typedef decltype nullptr  nullptr t      endif      LIBCPP HAS NO NULLPTR   endif      LIBCPP NULLPTR    a great deal can be uncovered with a quick grep -r  usr include        One thing that jumps out is the operator   overload  returning 0 is a lot friendlier than segfaulting      Another thing is it doesn t look compatible with storing an address at all   Which  compared to how it goes slinging void  s and passing NULL results to normal pointers as sentinel values  would obviously reduce the  never forget  it might be a bomb  factor

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Let me first give you an implementation of unsophisticated nullptr t  struct nullptr t        void operator amp    const   delete      Can t take address of nullptr      template lt class T gt      inline operator T    const   return 0         template lt class C  class T gt      inline operator T C      const   return 0        nullptr t nullptr    nullptr is a subtle example of Return Type Resolver idiom to automatically deduce a null pointer of the correct type depending upon the type of the instance it is assigning to   int  ptr   nullptr                    OK void  C   method ptr      nullptr     OK    As you can above  when nullptr is being assigned to an integer pointer  a int type instantiation of the templatized conversion function is created  And same goes for method pointers too  This way by leveraging template functionality  we are actually creating the appropriate type of null pointer every time we do  a new type assignment  As nullptr is an integer literal with value zero  you can not able to use its address which we accomplished by deleting  amp  operator    Why do we need nullptr in the first place    You see traditional NULL has some issue with it as below    1    Implicit conversion  char  str   NULL     Implicit conversion from void   to char   int i   NULL         OK  but  i  is not pointer type    2    Function calling ambiguity  void func int     void func int     void func bool     func NULL          Which one to call      Compilation produces the following error    error  call to  func  is ambiguous     func NULL            note  candidate function void func bool                                    note  candidate function void func int                                     note  candidate function void func int                                    1 error generated  compiler exit status 1    3    Constructor overload  struct String       String uint32 t            size of string             String const char              string                 String s1  NULL    String s2  5        In such cases  you need explicit cast  i e    String s  char  0

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Well  other languages have reserved words that are instances of types   Python  for instance    gt  gt  gt  None   5   File   lt stdin gt    line 1 SyntaxError  assignment to None  gt  gt  gt  type None   lt type  NoneType  gt    This is actually a fairly close comparison because None is typically used for something that hasn t been intialized  but at the same time comparisons such as None    0 are false   On the other hand  in plain C   NULL    0 would return true IIRC because NULL is just a macro returning 0  which is always an invalid address  AFAIK

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From nullptr  A Type-safe and Clear-Cut Null Pointer      The new C  09 nullptr keyword designates an rvalue constant that serves as a universal null pointer literal  replacing the buggy and weakly-typed literal 0 and the infamous NULL macro  nullptr thus puts an end to more than 30 years of embarrassment  ambiguity  and bugs  The following sections present the nullptr facility and show how it can remedy the ailments of NULL and 0    Other references    WikiBooks  with sample code  Here at Stack Overflow  Do you use NULL or 0  zero  for pointers in C    template Google group  comp lang c   moderated - compiler discussion

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It is a keyword because the standard will specify it as such   -  According to the latest public draft  n2914   2 14 7 Pointer literals  lex nullptr  pointer-literal  nullptr  The pointer literal is the keyword nullptr  It is an rvalue of type std  nullptr t   It s useful because it does not implicitly convert to an integral value

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How is it a keyword and an instance of a type    This isn t surprising  Both true and false are keywords and as literals they have a type   bool    nullptr is a pointer literal of type std  nullptr t  and it s a prvalue  you cannot take the address of it using  amp       4 10 about pointer conversion says that a prvalue of type std  nullptr t is a null pointer constant  and that an integral null pointer constant can be converted to std  nullptr t  The opposite direction is not allowed  This allows overloading a function for both pointers and integers  and passing nullptr to select the pointer version  Passing NULL or 0 would confusingly select the int version   A cast of nullptr t to an integral type needs a reinterpret cast  and has the same semantics as a cast of  void  0 to an integral type  mapping implementation defined   A reinterpret cast cannot convert nullptr t to any pointer type  Rely on the implicit conversion if possible or use static cast   The Standard requires that sizeof nullptr t  be sizeof void

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