C++ technical question

[BlasterBates]

Given this code:

template <class T> Kill (T *& ptr)
{
delete ptr;
ptr = NULL;

};

class MyClass{};

int main()
{
MyClass* myClass = new MyClass;
Kill(myClass);
Kill(myClass);
}

What should happen here?

With my (crappy) Microsoft compiler I executed this and it destructs twice when I have *& but generates an exception when I use * instead of *& in the template definition.

Surely when the pointer is set to NULL calling delete the second time should have no effect. Why was it executed twice with *&

Is it so that the above code should just execute without error and delete the object only once?

[VectraMan]

It'll call Kill() twice, but the second time ptr is NULL and so delete ptr will do nothing. Is that what you're saying is happening?

If Kill just takes a T* then ptr=NULL is only changing the local pointer, not the original, so the second call will try to delete an object that has already been deleted, and bang.

[d000hg]

Which 'crappy' MS compiler are we talking here?

Have you sanity-tested your code on any of these: https://www.google.co.uk/search?q=on...2B%2B+compiler

[BlasterBates]

Yes thx for that.

....of course a pointer to a pointer as it were.

That online compiler is very useful.

I´m using Microsoft Visual Studio C++ compiler which gives me that unexpected result of the destructor being called twice even though I set the ptr to NULL (when using *&).

[VectraMan]

Just tried it in VC8 for you, and the destructor is only called once. Perhaps the problem is with the other code you're not showing us?

You need a return type for the function (i.e. void) in VC8 otherwise it'll moan.

[BlasterBates]

It´s an old Beta version of Visual Studio, which doesn´t understand NULL and so set it to 0. Anyway it works with that online compiler. Results as expected. My faith in C++ has been restored.

gr8 stuff, thx.