What is NULL, does it need to be declared?

In this snippet I found here for a simple C++ hash everywhere there is a NULL I get a

NULL was not declared in this scope.

I'm using MinGW compiler g++. My guess is that NULL is not a reserved keyword? How could I determine the version of gcc and look at a reference list for C++ keywords?

This list here states that NULL is not a keyword.

int main()
  {
  }

const int TABLE_SIZE = 128;
class HashEntry 
  {
  private:
    int key;
    int value;
  public:
    HashEntry(int key, int value) 
      {
      this->key = key;
      this->value = value;
      }
    int getKey() 
      {
      return key;
      }
    int getValue() 
      {
      return value;
      }
  }; 
class HashMap 
  {
  private:
    HashEntry **table;
  public:
    HashMap() 
      {
      table = new HashEntry*[TABLE_SIZE];
      for (int i = 0; i < TABLE_SIZE; i++) table[i] = NULL;
      }
    int get(int key) 
      {
      int hash = (key % TABLE_SIZE);
      while (table[hash] != NULL && table[hash]->getKey() != key) hash = (hash + 1) % TABLE_SIZE;
      if (table[hash] == NULL)return -1;
      else return table[hash]->getValue();
开发者_开发百科      }
    void put(int key, int value) 
      {
      int hash = (key % TABLE_SIZE);
      while (table[hash] != NULL && table[hash]->getKey() != key)
      hash = (hash + 1) % TABLE_SIZE;
      if (table[hash] != NULL) delete table[hash];
      table[hash] = new HashEntry(key, value);
      }     
    ~HashMap() 
      {
      for (int i = 0; i < TABLE_SIZE; i++)
        if (table[i] != NULL) delete table[i];
      delete[] table;
      }
};

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NULL is a standard macro declared in stddef.h. You need to include stddef.h (or cstddef) in order to use NULL.

You can always use 0 in place of NULL, if you don't want to include anything.

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As mentioned elsewhere NULL is defined in stddef.h, and probably also in some other system header files (I have not checked this recently).

If you're using only C++ and not including anything then you can write:

#define NULL 0

at the top of your file and it will work as expected.

In newer C++0x or C++11 compilers have a keyword nullptr which should be used instead of NULL or 0 where pointers are concerned.