Why Does Invoking a Method on a Null Pointer in C Sometimes Work?

Understanding Undefined Behavior: Null Pointer Method Invocation in C

In C , the program execution can take unexpected turns when dealing with null pointers. This is evident in cases where a method is invoked through a null pointer, as exemplified in the following code snippet:

<code class="cpp">#include <iostream>

using namespace std;

class Test {
    int i;
public:
    Test() : i(0) { cout << "ctor called" << endl; }
    void show() { cout << "show fun called" << endl; }
};

int main(int argc, char *argv[]) {
    Test *ptr = NULL;
    ptr->show();
    return 0;
}</code>

Unexpectedly, the constructor function is not invoked despite the attempt to access the show() method through the null pointer ptr. This behavior stems from the fact that the compiler can determine the type of the null pointer, allowing it to identify the show() method. As the method doesn't utilize the this pointer, the code executes smoothly.

While convenient, this behavior is considered undefined by the C standard. The compiler is at liberty to generate code that can behave differently or crash the program. Consequently, relying on such behavior is unwise and may lead to unpredictable outcomes.

Therefore, it is crucial to avoid invoking methods through null pointers in C . Always ensure proper initialization of pointers to avoid undefined behavior and maintain program integrity.

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