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Re: New C++ warning about comparing this==NULL
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Re: New C++ warning about comparing this==NULL


  • Subject: Re: New C++ warning about comparing this==NULL
  • From: Anders Montonen <email@hidden>
  • Date: Thu, 03 Dec 2015 11:10:21 +0200

> 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? 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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  • Follow-Ups:
    • Re: New C++ warning about comparing this==NULL
      • From: Clark Cox <email@hidden>
References: 
 >New C++ warning about comparing this==NULL (From: Jens Alfke <email@hidden>)
 >Re: New C++ warning about comparing this==NULL (From: Anders Montonen <email@hidden>)
 >Re: New C++ warning about comparing this==NULL (From: Jens Alfke <email@hidden>)

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