Assigning general classes as null in C-CodePudding

I am having kind of a trouble adapting a program from C# to C . It's very commom given a class to assign it the null value. But C is not accepting the equivalent form 'nullptr'

class Point{
     public:
         int x,y;
}
//...
Point p = nullptr;

There is some way of solving it?

CodePudding user response：

The problem is that p is a non-pointer type in your example. This means that it cannot be initialized with a nullptr. The type of p is Point.

You can make p a pointer to a Point so that you can initialize it with a nullptr as shown below:

Point *p = nullptr; //p is a pointer to non-const Point object

If you want to create an object of the class type Point then one way of doing so is as follows:

Point p{1, 3};

I would also recommend referring to a good C book for a better understanding of the concept.

CodePudding user response：

You cannot assign nullptr to a class because a class is a type and not a variable. What you're doing by typing Point p = nullptr; is that you're asking the compiler to find a constructor of Point which accepts a nullptr.

You can either create an object of Point as an automatic variable by doing this:

Point p = Point{1, 2}; // sets x = 2 and y = 3

or simply like this:

Point p;

for which however you will need to define a constructor which initializes x and y because you haven't provided the member variables with default values.

Depending on the context in which this statement resides, p will either be allocated on the heap or on the stack. In case you're instantiating p in the main function, it will be the latter.

You can create a pointer to an object of Point by doing this:

Point* p = nullptr;

However, you will only have created a pointer. You will need to further take care of allocation, which you can do like this:

Point* p = new Point{1, 2};

in which case you should also free the allocated memory before your program ends like so:

delete p;