passing a NULL as an argument in a function

Question

i created a book class which i having being working on as part of my assignment but its seems have being having one problem which i am failing to understand in my code below, this is my code

private:
Book (string N = " ", int p = 100, string A = "",
        string P = "", string T = "", int Y = 2000)
{
    cout << "Book constructor start " << N << endl;
    Title=N;
    pages=p;
    Author=A;
    Publisher=P;
    Type=T;
    Yearpublished=Y;
}

~Book(void)
{
    cout << "Book destructor start " << Title << endl;
    system("pause");
}

public:
static Book * MakeBook(string N = "", int p = 100, string A = "",
        string P = "",string T = "",int Y = 2000)
{
    return new Book(N,p,A,P,T,Y);
}

static void DelBook(Book * X) {
    delete X;
}

In the above code is a constructor and destructor, my question is what happens when I pass a NULL as an argument in the stactic void DelBook function? like this below

static void DelBook(NULL)
{
    delete NULL;
}

How can I make it compile if its possible to pass a NULL value? Thanks in advance.

Answer 1

As long as DelBook only invokes delete — nothing happens, it's a no-op. (and it is possible to invoke your DelBook with NULL as a parameter value, no extra action needed).

Answer 2

NULL is a valid pointer: it points to memory location zero. Therefore you can pass NULL into a function that takes a Book* pointer, because it IS a Book* pointer.

Additionally, NULL is special and doesn't need to be cast to Book* - you do not need to say DelBook((Book*)NULL) . Therefore, the program should compile already.

Deleting a null pointer does nothing, so you don't need a sanity check. However if you need to do something with a member of the Book class, you must check it is not NULL first:

static void DelBook(Book * X)
{
    if (X){
        x->tidyUpBeforeDeletion();
        delete X;
    }
}

Failure to check this will result in a segfault - this is dereferencing a null pointer and is very bad news.

Answer 3

Passing NULL itself usually isn't a problem. It's basically 0. Deleting NULL probably has no effect