Checking for a null object in C++

I've mostly only worked with C and am running into some unfamiliar issues in C++.

Let's say that I have some function like this in C, which would be very typical:

int some_c_function(const char* var)
{
    if (var == NULL) {
        /* Exit early so we don't dereference a null pointer */
    }
    /* The rest of the code */
}

And let's say that I'm trying to write a similar function in C++:

int some_cpp_function(const some_object& str)
{
    if (str == NULL)  // This doesn't compile, probably because some_object doesn't overload the == operator

    if (&str == NULL) // This compiles, but it doesn't work, and does this even mean anything?
}

Basically, all I'm trying to do is to prevent the program from crashing when some_cpp_function() is called with NULL.

  • What is the most typical/common way of doing this with an object C++ (that doesn't involve overloading the == operator)?

  • Is this even the right approach? That is, should I not write functions that take an object as an argument, but rather, write member functions? (but even if so, please answer the original question)

  • Between a function that takes a reference to an object, or a function that takes a C-style pointer to an object, are there reasons to choose one over the other?

Answers


Basically, all I'm trying to do is to prevent the program from crashing when some_cpp_function() is called with NULL.

It is not possible to call the function with NULL. One of the purpose of having the reference, it will point to some object always as you have to initialize it when defining it. Do not think reference as a fancy pointer, think of it as an alias name for the object itself. Then this type of confusion will not arise.


A reference can not be NULL. The interface makes you pass a real object into the function.

So there is no need to test for NULL. This is one of the reasons that references were introduced into C++.

Note you can still write a function that takes a pointer. In this situation you still need to test for NULL. If the value is NULL then you return early just like in C. Note: You should not be using exceptions when a pointer is NULL. If a parameter should never be NULL then you create an interface that uses a reference.


As everyone said, references can't be null. That is because, a reference refers to an object. In your code:

// this compiles, but doesn't work, and does this even mean anything?
if (&str == NULL)

you are taking the address of the object str. By definition, str exists, so it has an address. So, it cannot be NULL. So, syntactically, the above is correct, but logically, the if condition is always going to be false.

About your questions: it depends upon what you want to do. Do you want the function to be able to modify the argument? If yes, pass a reference. If not, don't (or pass reference to const). See this C++ FAQ for some good details.

In general, in C++, passing by reference is preferred by most people over passing a pointer. One of the reasons is exactly what you discovered: a reference can't be NULL, thus avoiding you the headache of checking for it in the function.


A C++ reference is not a pointer nor a Java/C# style reference and cannot be NULL. They behave as if they were an alias to another existing object.

In some cases, if there are bugs in your code, you might get a reference into an already dead or non-existent object, but the best thing you can do is hope that the program dies soon enough to be able to debug what happened and why your program got corrupted.

That is, I have seen code checking for 'null references' doing something like: if ( &reference == 0 ), but the standard is clear that there cannot be null references in a well-formed program. If a reference is bound to a null object the program is ill-formed and should be corrected. If you need optional values, use pointers (or some higher level construct like boost::optional), not references.


You can use a special designated object as the null object in case of references as follows:

class SomeClass
{
    public:

        int operator==(SomeClass &object)
        {
            if(this == &object) 
            {
                    return true;
            }

            return false;
        }


    static SomeClass NullObject;
};

SomeClass SomeClass::NullObject;

void print(SomeClass &val)
{
    if(val == SomeClass::NullObject)
    {
        printf("\nNULL");
    }
    else
    {
        printf("\nNOT NULL");
    }
}

You should use NULL only with pointers. Your function accepts a reference and they can't be NULL.

Write your function just like you would write it in C.


C++ references naturally can't be null, you don't need the check. The function can only be called by passing a reference to an existing object.


  • What is the most typical/common way of doing this with an object C++ (that doesn't involve overloading the == operator)?
  • Is this even the right approach? ie. should I not write functions that take an object as an argument, but rather, write member functions? (But even if so, please answer the original question.)

No, references cannot be null (unless Undefined Behavior has already happened, in which case all bets are already off). Whether you should write a method or non-method depends on other factors.

  • Between a function that takes a reference to an object, or a function that takes a C-style pointer to an object, are there reasons to choose one over the other?

If you need to represent "no object", then pass a pointer to the function, and let that pointer be NULL:

int silly_sum(int const* pa=0, int const* pb=0, int const* pc=0) {
  /* Take up to three ints and return the sum of any supplied values.

  Pass null pointers for "not supplied".

  This is NOT an example of good code.
  */
  if (!pa && (pb || pc)) return silly_sum(pb, pc);
  if (!pb && pc) return silly_sum(pa, pc);
  if (pc) return silly_sum(pa, pb) + *pc;
  if (pa && pb) return *pa + *pb;
  if (pa) return *pa;
  if (pb) return *pb;
  return 0;
}

int main() {
  int a = 1, b = 2, c = 3;
  cout << silly_sum(&a, &b, &c) << '\n';
  cout << silly_sum(&a, &b) << '\n';
  cout << silly_sum(&a) << '\n';
  cout << silly_sum(0, &b, &c) << '\n';
  cout << silly_sum(&a, 0, &c) << '\n';
  cout << silly_sum(0, 0, &c) << '\n';
  return 0;
}

If "no object" never needs to be represented, then references work fine. In fact, operator overloads are much simpler because they take overloads.

You can use something like boost::optional.


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