A reference can not be NULL or it can be NULL?

Question

I have read from the Wikipedia that:

“References cannot be null, whereas pointers can; every reference refers to some object, although it may or may n

Answer 1

that would crash your program. Did you try running it? doing *object will deference a null pointer, so in fact your reference never gets assigned.

Answer 2

Well, you can do whatever you want in C++. Another example:

person &object1 = *( reinterpret_cast<person*>(0) );

You are invoking an undefined behavior in the above case, beside the case you mentioned!

Answer 3

Saying person &object1=*object is not the same thing as saying person &object1=NULL. Probably the compiler is just not smart enough to find out that you are dereferencing null pointer, but you'll get a runtime error anyway. So they are kind of true still ;)

Answer 4

In your code:

person *object=NULL;
person &object1=*object;

you dereference a NULL pointer, so you get undefined behaviour. And to answer your question, there is no such thing as a NULL reference.

And to address the other part of your question, just because a program compiles, there is no guarantee that it is correct or that it will work. C++ compilers are not required to even attempt to diagnose the kind of error your code contains.

Answer 5

You can have a null reference, not sure why anyone would say otherwise, it is a nasty side effect of some operations. You just can't create one directly.

Answer 6

gcc8 will give a warning about it:

warning: the compiler can assume that the address of 'object1' will never be NULL [-Waddress]

A small demo:

#include <iostream>

class person
{
    public:
        virtual void setage()=0;
};

int main()
{
    person *object=NULL;
    person &object1=*object;

    if (&object1 == NULL) {
        std::cout << "NULL object1" << std::endl;
    }

    if (!(&object1)) {
        std::cout << "NULL object1 " << std::endl;
    }
}

Compile and running output:

g++ -std=c++2a -pthread -fgnu-tm -O2 -Wall -Wextra -pedantic -pthread -pedantic-errors main.cpp -lm -latomic -lstdc++fs && ./a.out

main.cpp: In function 'int main()':

main.cpp:14:18: warning: the compiler can assume that the address of 'object1' will never be NULL [-Waddress]

 if (&object1 == NULL) {

              ^

main.cpp:18:19: warning: the compiler can assume that the address of 'object1' will never be NULL [-Waddress]

 if (!(&object1)) {

               ^

NULL object1

NULL object1