> On Dec 3, 2015, at 01:10, Anders Montonen <[email protected]> wrote:
>
>
>> On 03 Dec 2015, at 1:43, Jens Alfke <[email protected]> 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
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