What Happens When You Call a Member Function on a Null Object Pointer?

Incurring Undefined Behavior When Calling a Member Function on a Null Object Pointer

A common interview question probes the potential consequences of invoking a member function on a null object pointer. This scenario can lead to undefined behavior, leaving the program open to unpredictable outcomes.

Let's examine the following code:

class A
{
public:
    void fun()
    {
        std::cout << "fun" << std::endl;
    }
};

A* a = NULL;
a->fun();

Calling the fun() method on the null pointer a triggers undefined behavior. Here's why:

When creating a pointer like a, it's essentially a memory address. Setting it to NULL indicates that this pointer doesn't point to any valid object. By accessing a->fun(), the program attempts to dereference the NULL pointer, which is forbidden.

Since the object pointed to by a doesn't exist, the program may exhibit unpredictable behavior, such as:

• Program crash: The processor may encounter an access violation, causing the program to terminate.
• Incorrect output: The program might print "fun" since the method doesn't rely on object member variables. However, this relies on the memory pointed to by a not being corrupted, which isn't always guaranteed.
• Other undefined behavior: Anything can happen, including memory corruption or unexpected exceptions.

To prevent such undefined behavior, it's crucial to ensure that object pointers are valid before dereferencing them.

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