Checking for a null object in C++


Question

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?

1
32
1/27/2016 1:25:38 PM

Accepted Answer

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.

34
1/20/2010 9:14:37 AM

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.


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