setting equal to NULL

This is a discussion on setting equal to NULL within the C++ Programming forums, part of the General Programming Boards category; When I write this code, I get an error from my compiler saying that NULL is an undeclared identifier. Code: ...

  1. #1
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    Question setting equal to NULL

    When I write this code, I get an error from my compiler saying that NULL is an undeclared identifier.

    Code:
    #ifndef DISK_H
    #define DISK_H
    
    class Disk
    {
    public:
    	Disk(int no) : number(no), next(NULL) {}
    	~Disk() {}
    
    	bool operator <(const Disk& rhs);
    	bool operator >(const Disk& rhs);
    	bool operator ==(int rhs);
    
    	int reveal_number()
    	{
    		return this->number;
    	}
    
    private:
    	int number;
    	Disk* next;
    };
    
    #endif
    I've also tried setting next equal to null inside the curly brackets instead, but that has the same problem. What's the problem?

  2. #2
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  3. #3
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    Thx. I'm still curious as to how it goes wrong though.

  4. #4
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    NULL is not a keyword, it is a symbol that is defined somewhere. At the point when the compiler is compiling your code, it has not seen that definition (or rather, the pre-compiler has not replaced NULL with 0 because it hasn't seen the definition).

    NULL is defined according to the standard in a specific header (I forget which one), and on some platforms in some cases it tends to work even if you don't include that specific header. You just happen to have hit a situation where it doesn't work.

  5. #5
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    I'm still curious as to how it goes wrong though.
    If you really want to use NULL instead of 0, you should #include <cstddef>
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  6. #6
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    Thank y'all. As for what y'all said about needing to pull out a header for it to not be a problem, usually I will include iostream and probably string, and it appears that iostream has definition of it as well as cstddef. thx.

  7. #7
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    In some (or even many) cases that might be true, but usually that is something you don't want to rely on because if you ever move your code to another platform (or post it for somebody else to try) then it suddenly won't work.

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