Why is NULL undeclared

Question

I have a problem with this struct contructor when I try to compile this code   typedef struct Node       Node  int data                    this- gt data   data          previous   NULL     Compiler indicates here         next   NULL             int data      Node  previous      Node  next    NODE    when I come this error occurs    linkedlist linkedlist h  In constructor  Node  Node int      linkedlist linkedlist h 9 error   NULL  was not declared in this scope            Build finished  1 errors  0 warnings        Last problem was the struct  but it worked fine when it was in my main cpp  this time it s in a header file and is giving me this problem  I am using Code  Blocks to compile this code

User · Answer

Do use NULL  It is just  defined as 0 anyway and it is very useful to semantically distinguish it from the integer 0   There are problems with using 0  and hence NULL   For example   void f int   void f void     f 0      Ambiguous  Calls f int     The next version of C    C  0x  includes nullptr to fix this   f nullptr      Calls f void

User · Answer

Are you including  stdlib h  or  cstdlib  in this file   NULL is defined in stdlib h cstdlib   include  lt stdlib h gt    or   include  lt cstdlib gt      This is preferrable for c

User · Answer

NULL is not a built-in constant in the C or C   languages  In fact  in C   it s more or less obsolete  just use a plain literal 0 instead  the compiler will do the right thing depending on the context   In newer C    C  11 and higher   use nullptr  as pointed out in a comment  thanks    Otherwise  add   include  lt stddef h   to get the NULL definition

User · Answer

NULL isn t a native part of the core C   language  but it is part of the standard library  You need to include one of the standard header files that include its definition   include  lt cstddef gt  or  include  lt stddef h gt  should be sufficient   The definition of NULL is guaranteed to be available if you include cstddef or stddef h   It s not guaranteed  but you are very likely to get its definition included if you include many of the other standard headers instead

User · Answer

Don t use NULL  C   allows you to use the unadorned 0 instead   previous   0  next   0    And  as at C  11  you generally shouldn t be using either NULL or 0 since it provides you with nullptr of type std  nullptr t  which is better suited to the task

[c++] Why is NULL undeclared?

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