new char(10) is wrong, but what was it doing?

[dugeen]

I spent a long time puzzling over a crashing program and realised that I'd put

ptr = new char(10);

where I should have had

ptr = new char[10];

But I noticed that the wrong version didn't make the compiler issue a warning. So new char(10) must have a meaning? Is it a construction you'd ever use purposely?

[hanst99]

That's placement new - in this case it would be equal to

ptr = (char*)10;

so that returns a char* with the value 10. It assumes you already own that memory.

If you'd do that with a class the constructor of that class would also be called.

The only reason I can think of right now why one would use this is if you use a memory pool. Normally that's not the case, so you can probably just ignore the existence of that for now.

[Disch]

That's placement new

Err... no it's not.

It's creating one int with new and initializing it with the value of 10.

Similar to this:

ptr = new int;
*ptr = 10;

EDIT:

as for why you'd use it, it's more useful with classes than basic types because it allow you to call a specific constructor:

// some class's constructor:
SomeClass::SomeClass(int a, int b, int c) { }

// calling that ctor with new:
SomeClass* foo = new SomeClass(1,2,3);

[dugeen]

Now I've seen your example with a class name, it seems obvious. I can't work out why I couldn't work it out! Thanks.

[hanst99]

Stupid me. Yeah sorry disch, I got stuff like that sometimes. I shouldn't make posts after 8pm anymore, I just don't function.