空类指针中的方法（C ++）

Question

Let's say whe have

class Foo{

public:
       bool   error;
       ......
       bool isValid(){return error==false;}
};

and somewhere

Foo *aFoo=NULL;

I usually would do if (aFoo!=NULL && aFoo->isValid()).....

But what if in the isValid method I test the nullity:

bool isValid(){return this!=NULL && error==false)

That would simplify the external testing with simply calling if (aFoo->isValid())

I've tested it in some compilers and it works but I wonder if it is standard and could cause problems when porting to other environments.

Answer 1

The compiler is free to optimize away the check -- calling any non-static member of any class through an invalid (or NULL pointer) is undefined behavior. Please don't do this.

Answer 2

Generally it would be bad design and in standard C++ it doesn't make much sense as your internal NULL check implies that you would call a null pointer, which is undefined behavior.

This topic was discusses here: Checking if this is null

Answer 3

Why not simply a namespace-scope function like this?

bool isValid(Foo* f) {return f && f->isValid();}

An if -Statement like

if (aFoo->isValid())

Implies that the pointer is pointing to a valid object. It would be a huge source of confusion and very error prone.

Finally, your code would indeed invoke undefined behavior - aFoo->isValid is per definition equivalent to (*aFoo).isValid :

N3337, §5.2.5/2

The expression E1->E2 is converted to the equivalent form (*(E1)).E2 ;

which would dereference a null pointer to obtain a null reference, which is clearly undefined:

N3337, §8.3.2/5

[ Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer, which causes undefined behavior. […] — end note ]