Is null reference possible

Question

Is this piece of code valid  and defined behavior    int  amp nullReference     int  0    Both g   and clang   compile it without any warning  even when using -Wall  -Wextra  -std c  98  -pedantic  -Weffc       Of course the reference is not actually null  since it cannot be accessed  it would mean dereferencing a null pointer   but we could check whether it s null or not by checking its address   if   amp  nullReference    0      null reference

User · Answer

clang   3 5 even warns on it    tmp a C 3 7  warning  reference cannot be bound to dereferenced null pointer in well-defined C   code  comparison may be assumed to       always evaluate to false  -Wtautological-undefined-compare  if   amp  nullReference    0      null reference                          1 warning generated

User · Answer

If your intention was to find a way to represent null in an enumeration of singleton objects  then it s a bad idea to  de reference null  it C  11  nullptr    Why not declare static singleton object that represents NULL within the class as follows and add a cast-to-pointer operator that returns nullptr    Edit  Corrected several mistypes and added if-statement in main   to test for the cast-to-pointer operator actually working  which I forgot to   my bad  - March 10 2015 -     Error h class Error   public    static Error amp  NOT FOUND    static Error amp  UNKNOWN    static Error amp  NONE     singleton object that represents null  public    static vector lt shared ptr lt Error gt  gt   instances    static Error amp  NewInstance const string amp  name  bool isNull   false    private    bool  isNull    Error const string amp  name  bool isNull   false     name name    isNull isNull        Error         Error const Error amp  src        Error amp  operator  const Error amp  src       public    operator Error      return  isNull   nullptr   this           Error cpp vector lt shared ptr lt Error gt  gt  Error   instances  Error amp  Error  NewInstance const string amp  name  bool isNull   false      shared ptr lt Error gt  pNewInst new Error name  isNull      Error   instances push back pNewInst     return  pNewInst get       Error amp  Error  NOT FOUND   Error  NewInstance  NOT FOUND      Error amp  Error  NOT FOUND   Error  NewInstance  UNKNOWN    Edit  fixed   Error amp  Error  NOT FOUND   Error  NewInstance  NONE   true   Edit  fixed Error amp  Error  UNKNOWN   Error  NewInstance  UNKNOWN    Error amp  Error  NONE   Error  NewInstance  NONE        Main cpp  include  Error h   Error amp  getError       return Error  UNKNOWN        Edit  To see the overload of  Error     in Error h actually working Error amp  getErrorNone       return Error  NONE     int main void      if getError      Error  NONE        return EXIT FAILURE            Edit  To see the overload of  Error     in Error h actually working   if getErrorNone      nullptr        return EXIT FAILURE

User · Answer

Yes   include  lt iostream gt   include  lt functional gt   struct null ref t      template  lt typename T gt     operator T amp            union TypeSafetyBreaker            T  ptr               see https   stackoverflow com questions 38691282 use-of-union-with-reference          std  reference wrapper lt T gt  ref                   TypeSafetyBreaker ptr     ptr   nullptr             unwrap the reference       return ptr ref get             null ref t nullref    int main        int  amp a   nullref         Segmentation fault    a   4     return 0

User · Answer

The answer depends on your view point   If you judge by the C   standard  you cannot get a null reference because you get undefined behavior first  After that first incidence of undefined behavior  the standard allows anything to happen  So  if you write   int  0  you already have undefined behavior as you are  from a language standard point of view  dereferencing a null pointer  The rest of the program is irrelevant  once this expression is executed  you are out of the game   However  in practice  null references can easily be created from null pointers  and you won t notice until you actually try to access the value behind the null reference  Your example may be a bit too simple  as any good optimizing compiler will see the undefined behavior  and simply optimize away anything that depends on it  the null reference won t even be created  it will be optimized away   Yet  that optimizing away depends on the compiler to prove the undefined behavior  which may not be possible to do  Consider this simple function inside a file converter cpp  int amp  toReference int  pointer        return  pointer     When the compiler sees this function  it does not know whether the pointer is a null pointer or not  So it just generates code that turns any pointer into the corresponding reference   Btw  This is a noop since pointers and references are the exact same beast in assembler   Now  if you have another file user cpp with the code  include  quot converter h quot   void foo         int amp  nullRef   toReference nullptr       cout  lt  lt  nullRef       crash happens here    the compiler does not know that toReference   will dereference the passed pointer  and assume that it returns a valid reference  which will happen to be a null reference in practice  The call succeeds  but when you try to use the reference  the program crashes  Hopefully  The standard allows for anything to happen  including the appearance of pink elephants  You may ask why this is relevant  after all  the undefined behavior was already triggered inside toReference    The answer is debugging  Null references may propagate and proliferate just as null pointers do  If you are not aware that null references can exist  and learn to avoid creating them  you may spend quite some time trying to figure out why your member function seems to crash when it s just trying to read a plain old int member  answer  the instance in the call of the member was a null reference  so this is a null pointer  and your member is computed to be located as address 8    So how about checking for null references  You gave the line if   amp  nullReference    0      null reference  in your question  Well  that won t work  According to the standard  you have undefined behavior if you dereference a null pointer  and you cannot create a null reference without dereferencing a null pointer  so null references exist only inside the realm of undefined behavior  Since your compiler may assume that you are not triggering undefined behavior  it can assume that there is no such thing as a null reference  even though it will readily emit code that generates null references    As such  it sees the if   condition  concludes that it cannot be true  and just throw away the entire if   statement  With the introduction of link time optimizations  it has become plain impossible to check for null references in a robust way   TL DR  Null references are somewhat of a ghastly existence  Their existence seems impossible    by the standard   but they exist    by the generated machine code   but you cannot see them if they exist    your attempts will be optimized away   but they may kill you unaware anyway    your program crashes at weird points  or worse   Your only hope is that they don t exist    write your program to not create them   I do hope that will not come to haunt you

User · Answer

References are not pointers   8 3 2 1      A reference shall be initialized to   refer to a valid object or function     Note  in particular  a null reference   cannot exist in a well-defined   program  because the only way to   create such a reference would be to   bind it to the    object    obtained by   dereferencing a null pointer  which   causes undefined behavior  As   described in 9 6  a reference cannot   be bound directly to a bit-field      1 9 4      Certain other operations are described   in this International Standard as   undefined  for example  the effect of   dereferencing the null pointer    As Johannes says in a deleted answer  there s some doubt whether  dereferencing a null pointer  should be categorically stated to be undefined behavior  But this isn t one of the cases that raise doubts  since a null pointer certainly does not point to a  valid object or function   and there is no desire within the standards committee to introduce null references

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