I just saw this beauty today...

N a v a n e e t h

In C++ that makes sense as this can be NULL in some situations. :)

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Dan Neely

My c++ is obviously rusty, what situations are those?

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WilliamSauron

Call a method passing a null object... In Delphi, the Free method checks that it was called for an object that was really created. It's quite useful, since you can write "MyObject.Free" instead of "if Assigned(MyObject) then MyObject.Free;". A real time saver, and code is so much more readable.

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Dan Neely

C++ lets you set the this object in the parameters instead of always using the object the method is called on?

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WilliamSauron

No, but if you write:

int foo ()
{
string *s = null;
return s->length();
}

the string::length function will be called with this==null. This could be used so that the null string would behave as the empty string.

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Dan Neely

OK, that makes sense.

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N a v a n e e t h

William has already explained it neatly. Here is another example if you are interested.

class Dummy{
public:
void CheckNull(){
if(!this)
std::cout << "This is NULL!";
}
};

Dummy* dummy = 0;
dummy->CheckNull();

You can see This is NULL message appearing in the above example. Above code is problematic and as per C++ standard, calls on an uninitialized pointer will lead into unexpected results.

dan neely wrote:

C++ lets you set the this object in the parameters instead of always using the object the method is called on?

No. AFAIK, compiler compiles the CheckNull() method like

void Dummy_CheckNull(Dummy* this);

and the Dummy instance we created will be passed to this method. Your code will work fine if you are not using the this. Any operation on this will fail as it is not initialized. Hope it is clear now! :)

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Jim Crafton

Thereby obscuring the real problem, which is that you have a null object instance you're trying to use. Loads of fun tracking down something like that.

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TheScientistIsDead

would this be another example?

class NullCall {
public:
char PublicMethod(int x) {
return (char)x; }
};

NullCall::PublicMethod(12);

I.E. using the namespace/scope operator '::' ? -Adam

N a v a n e e t h

TheScientistIsDead wrote:

would this be another example?

I don't think so. Your code produces error " error: cannot call member function ‘char NullCall::PublicMethod(int)’ without object". You can compile this by making the method as static but you can't use this inside a static method. :)

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