I have a problem with this struct contructor when I try to compile this code:
typedef struct Node
{
Node( int data ) //
{
this->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
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 <stddef.h>
to get the NULL
definition.
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.
Are you including "stdlib.h" or "cstdlib" in this file? NULL is defined in stdlib.h/cstdlib
#include <stdlib.h>
or
#include <cstdlib> // This is preferrable for c++
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 <cstddef>
or #include <stddef.h>
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.
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*).
Source: Stackoverflow.com