Re: New C++ warning about comparing this==NULL
Re: New C++ warning about comparing this==NULL
- Subject: Re: New C++ warning about comparing this==NULL
- From: Clark Cox <email@hidden>
- Date: Thu, 03 Dec 2015 17:17:17 -0800
> On Dec 3, 2015, at 01:10, Anders Montonen <email@hidden> wrote:
>
>
>> On 03 Dec 2015, at 1:43, Jens Alfke <email@hidden> wrote:
>>
>> If x is a value of class/struct X, and y is a non-virtual method, then x.y() turns into X::y(&x).
>>
>> So putting the two together, if x is a pointer to X, then x->y() turns into X::y(x). There’s no pointer dereference involved, and no issue if x is NULL. All that happens is that in the implementation of y(), `this` will be NULL.
>
> Is that standard-mandated behaviour or an optimization?
It is a legal optimization due to the fact that it is illegal to call a member function through a NULL pointer. i.e. no valid C++ code can ever result in “this" being NULL, so the compiler is free to assume that "this == NULL" is always false.
> If it is legal for any member function call to go via the this pointer, then the check is illegal. As a practical consideration, in this example (modified from the viva64 post to add a data member to the second base class), Xcode 7.1.1 produces a non-null this pointer from a null pointer call:
>
> #include <stdio.h>
>
> class FirstBase {
> int firstBaseData;
> };
>
> class SecondBase {
> int secondBaseData;
> public:
> void Method()
> {
> if( this == 0 ) {
> printf( "this == 0\n");
> } else {
> printf( "this != 0 (value: %p)\n", this );
> }
> }
> };
>
> class Composed1 : public FirstBase, public SecondBase {
> };
>
> int main()
> {
> Composed1* object = 0;
> object->Method();
> }
>
> -a
> _______________________________________________
> Do not post admin requests to the list. They will be ignored.
> Xcode-users mailing list (email@hidden)
> Help/Unsubscribe/Update your Subscription:
>
> This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Xcode-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden