Is it possible to set an object to null?

问题:

Further in my code, I check to see check if an object is null/empty.

Is there a way to set an object to null?

回答1:

An object of a class cannot be set to NULL; however, you can set a pointer (which contains a memory address of an object) to NULL.

Example of what you can't do which you are asking:

Cat c;
c = NULL;//Compiling error

Example of what you can do:

Cat c;
//Set p to hold the memory address of the object c
Cat *p = &c;
//Set p to hold NULL
p = NULL;

回答2:

While it is true that an object cannot be "empty/null" in C++, in C++17, we got std::optional to express that intent.

Example use:

std::optional<int> v1;      // "empty" int
std::optional<int> v2(3);   // Not empty, "contains a 3"

You can then check if the optional contains a value with

v1.has_value(); // false

or

if(v2) {
    // You get here if v2 is not empty
}

A plain int (or any type), however, can never be "null" or "empty" (by your definition of those words) in any useful sense. Think of std::optional as a container in this regard.

If you don't have a C++17 compliant compiler at hand, you can use boost.optional instead. Some pre-C++17 compilers also offer std::experimental::optional, which will behave at least close to the actual std::optional afaik. Check your compiler's manual for details.

回答3:

You want to check if an object is NULL/empty. Being NULL and empty are not the same. Like Justin and Brian have already mentioned, in C++ NULL is an assignment you'd typically associate with pointers. You can overload operator= perhaps, but think it through real well if you actually want to do this. Couple of other things:

1. In C++ NULL pointer is very different to pointer pointing to an 'empty' object.
2. Why not have a bool IsEmpty() method that returns true if an object's variables are reset to some default state? Guess that might bypass the NULL usage.
3. Having something like A* p = new A; ... p = NULL; is bad (no delete p) unless you can ensure your code will be garbage collected. If anything, this'd lead to memory leaks and with several such leaks there's good chance you'd have slow code.
4. You may want to do this class Null {}; Null _NULL; and then overload operator= and operator!= of other classes depending on your situation.

Perhaps you should post us some details about the context to help you better with option 4.

Arpan

回答4:

You can set any pointer to NULL, though NULL is simply defined as 0 in C++:

myObject *foo = NULL;

Also note that NULL is defined if you include standard headers, but is not built into the language itself. If NULL is undefined, you can use 0 instead, or include this:

#ifndef NULL
#define NULL 0
#endif

As an aside, if you really want to set an object, not a pointer, to NULL, you can read about the Null Object Pattern.

回答5:

"an object" of what type?

You can certainly assign NULL (and nullptr) to objects of pointer types, and it is implementation defined if you can assign NULL to objects of arithmetic types.

If you mean objects of some class type, the answer is NO (excepting classes that have operator= accepting pointer or arithmetic types)

"empty" is more plausible, as many types have both copy assignment and default construction (often implicitly). To see if an existing object is like a default constructed one, you will also need an appropriate bool operator==