New and delete for multiple instances

[gab74]

Hy all,

I've a simple project with 3 buttons
the first and second to inizialize a Class
the third to delete pointer.


I declare a private variable

OtherClass *a;



in first method call

a = new OtherClass(5);


in Second method call

a = new OtherClass(9);


in Third method i want to delete pontier

I'm wandering

delete a;

will work ??

All memory allocated for the two new statements is free ??
I know that i can use a single pointer, but i want to know if there is a way
to call new statement on a class many times, and a way to deallocate memory..

Thanks in advance

[MikeyBoy]

You're need to think logically about what you're doing.

At:

a = new OtherClass(5);

you're setting the value of a to store the address of the object you've created.

Then, at:

a = new OtherClass(9);

you're changing the value of a to store the address if the second object you've created.

So when you do delete a you'll be deleting whichever object has its address currently stored in a.

a can't magically hold multiple values at once. It's just like any other variable - you give it a value, and you can change that value, but it's just a single variable.

[gab74]

OK perfect !! Thank you for your precius support !!!

So when i delete a i will delete the the single variable !! And there are only one variable !

A last question

to delete a

can i use

if(a != NULL) [ better if(a) ]
delete a ;

Is this correct ??

[keskiverto]

The standard says that "delete 0;" does properly nothing. Therefore, it is safe to delete a null pointer.
Consider delete a; a = 0;
and

if ( a )
{
  a->setValue( 42 );
}
else
{
  a = new OtherClass( 42 );
}

[MikeyBoy]

So when i delete a i will delete the the single variable !! And there are only one variable

Yes. If it helps, think of a pointer as a number. And that number is an address in memory, where the OtherClass object is stored.

A last question

to delete a

can i use

if(a != NULL) [ better if(a) ]
delete a ;

Yes, that works. You probably don't need the if statement - in most C++ implementations, it's harmless doing delete NULL; anyway, so it won't matter.

I can't remember what the C++ standard says about doing delete NULL - anyone?
Edit: keskiverto answered my question before I'd even finished asking it :)

[gab74]

Thanks to everybody for precious description !!
Topic solved !