Null object in c++

Question

i am trying to check whether the ship object is null, but i got an error message

Crane.cpp:18: error: could not convert ‘((Crane*)this)->Crane::ship.Ship::operator=(((const Ship&)(& Ship(0, std::basic_string, std::allocator >(((const char*)"arrive"), ((const std::allocator&)((const std::allocator)(& std::allocator())))), std::basic_string, std::allocator >(((const char)"Ship"), ((const std::allocator&)((const std::allocator*)(& std::allocator()))))))))’ to ‘bool’

Crane::Crane(int craneId, int craneStatus, bool free, Ship ship)
{
    setCraneId(craneId);
    setCraneStatus(craneStatus);
    setFree(free);
    setShip(ship);
}
Crane::Crane(){}
Crane::~Crane(){}

void Crane::print()
{
    cout << "Crane Id: " << craneId << endl;
    cout << "Crane Status: " << craneStatus << endl;
    cout << "Crane is free: " << free << endl;
    if (ship = NULL) //this is the problem
    {
        cout << " " << endl;
    }
    else
    {
        ship.print();//i have another print method in the Ship class
    }
}

i have tried

if (ship == NULL)

but i get this error message

Crane.cpp:18: error: no match for ‘operator==’ in ‘((Crane*)this)->Crane::ship == 0’

how to do this right?

I guess you come from `C#`. `C#` works with references (not exactly the same as `C++` references) and those can be null while plain `C++` objects can't. — ereOn, Sep 07 '10 at 06:32
`if (ship = NULL)` looks like an error even if `ship` was a pointer. Note that a single `=` means assignment, not comparison. — David Rodríguez - dribeas, Sep 07 '10 at 08:03
You need to get [a book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and actually learn C++ if you're going to program C++. — GManNickG, Sep 07 '10 at 13:13

Cedric H. · Answer 1 · 2014-03-25T17:01:54.263

14

That's because ship is not a pointer to Ship, i.e. a Ship* but it is a Ship object itself; then you cannot convert it to 0 which is the null address for ... a pointer.

If you want a pointer to Ship you should do

Ship* ship = new Ship;
// Catch a std::bad_alloc exception if new fails

Then if you obtain the pointer as a function argument you can test if it is null or not:

void foo(Ship* ship_pointer)
{
    if(ship_pointer == 0)
        // Oops pointer is null...
    else
        // Guess it's OK and use it.
}

edited Mar 25 '14 at 17:01

answered Sep 07 '10 at 06:25

Cedric H.

7,980
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Except that if the line Ship* ship = new Ship fails you won't get a NULL pointer, you'll get a std::bad_alloc exception thrown. – hookenz Sep 07 '10 at 09:04
Yes, my two examples are in fact disconnected. – Cedric H. Sep 07 '10 at 09:31
Could you update this answer to avoid the owning raw pointer? – Baum mit Augen Dec 29 '17 at 16:48

score 3 · Answer 2 · answered Sep 07 '10 at 06:25

From the look of the code you have decalred ship as a member variable inside the class Crane. If it is so, it can not be NULL. When Crane object is created the ship object is also constructed. You can test for NULL only for pointers like that. I suggest you to read a book explaining the basic C++ syntax before proceeding further. If your intention is to check for whether ship is empty, you can provide a method called empty in ship class which returns a bool and use that in the if condition.

score 2 · Answer 3 · answered Sep 07 '10 at 06:30

how to do this right?

Easy: Don't treat C++ like Java or some other language that forces an indirection onto you (reference<->object). C++ doesn't work like that. If you want an indirection you have to use something like pointers. There is really no difference between a class type and an int or double with recpect to copying, assignment, etc. There is no implicit indirection with class types.

score 1 · Answer 4 · answered Sep 07 '10 at 06:36

if (ship = NULL)

You got 3 ways of doing it:

1) use comparison operator instead of assignment operator and overload Ship's operator== to take int - alternatively you can define global bool ::operator==(Ship const& ship, int)
2) create static nullship variable of type Ship, so you can use it for comparisons
3) the best thing you can compare to NULL is a pointer, especially in your case Ship*

And it is a good practice to take every argument by reference, so it's not copied each time you call the function.

Null object in c++

4 Answers4