Why Does Calling a Non-Virtual Member Function on a Null Pointer in C Sometimes Work?

Gotcha in C : Accessing Non-Virtual Class Members on a Null Pointer

In C , calling non-virtual member functions on a null pointer may unexpectedly work, posing a curious puzzle. Let's delve into this behavior through an intriguing code snippet:

Puzzle Explained

The confusion arises from the different mechanisms used to call virtual and non-virtual methods.

• Virtual methods: Require vtable lookup to determine the function to call, which fails for null pointers as they have no valid objects associated with them.
• Non-virtual methods: Are compiled directly as a function call with the this pointer (object reference) passed as a hidden parameter.

In the given code, the non-virtual method say_hi() never explicitly dereferences the this pointer, bypassing the null pointer dereference error. It's essentially the equivalent of a function call with a parameter passed by value:

Undefined Behavior

Technically, calling any function (even non-virtual) on a null pointer is undefined behavior in C . However, some compiler implementations may provide well-defined behavior for specific scenarios, such as the above non-virtual function call.

Caution

While the unexpected behavior may seem convenient, relying on it is risky and not a best practice. Always avoid accessing class members on null pointers, as it can lead to unexpected and unpredictable results.

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