Why does calling method through null pointer “work” in C++? [duplicate]

Answer 1

If you look at the assembly (for at least one compiler), you can see why it runs (even though it is undefined behavior as many have pointed out). For these two lines:

test *ptr = NULL;
ptr->show();

This assembly is generated (in one compiler I just tried):

00000004: C7 45 FC 00 00 00  mov         dword ptr [ebp-4],0
          00
0000000B: 8B 4D FC           mov         ecx,dword ptr [ebp-4]
0000000E: E8 00 00 00 00     call        ?show@test@@QAEXXZ

It pushes the NULL (0) on the stack and calls the method since the address of the method is independent of the actual object instance.

Answer 2

The pointer isn't needed to call the method. The type of the pointer is known, so the code for the method is known. The method doesn't use this, so it runs the code just fine. It's undefined behavior, but its more efficient not to check if the pointer is NULL, so it runs.

Answer 3

Well, first off, it's not valid as it invokes undefined behavior. You are really asking why the compiler allows it, and the answer is because that is boneheaded code that simply won't occur in a real application, so why bother? The real problem comes when the pointer is made invalid at runtime in a way that cannot be anticipated by static code analysis.

The compiler is not there to hold your hand, it is there to compile your code according to the standard. If it offers useful warnings then great, but there is nothing syntactically or semantically illegal there, you are simply writing code which causes undefined behavior.

Answer 4

It is not valid, the behavior is undefined and actual results depended on your compiler.