const attribute makes whole element const?

[Jonathan M Davis]

On Sunday, September 09, 2012 17:09:23 Namespace wrote:
> On Saturday, 8 September 2012 at 23:18:14 UTC, Timon Gehr wrote:
> > On 09/09/2012 01:16 AM, Namespace wrote:
> >> Why fail this code?
> >> without "const" on "Name" it works fine.
> >> 
> >> http://dpaste.dzfl.pl/9fa0986a
> > 
> > const fields cannot be written to. This includes the case when
> > the
> > entire struct is written to at once.
> 
> But i assign the const attribute in the ctor. So this behaviour
> doesn't make any sense...

const member variables can be initialized but never assigned to. They're 
either directly initialized or initialized by the constructor. After that 
(including later in the constructor), you can't change them. This also makes 
the default assignment operator illegal. You could overload it, and as long as 
it doesn't touch any of the const member variables, it would work, but the 
const member variable is stuck as it is, and anything trying to mutate is 
illegal.

Really, having const member variables is a _bad_ idea IMHO - particularly for 
structs. At least with a class, if you try and replace the object's value, you 
generally just allocate a new object and assign it to the reference. But with 
structs, which are almost always on the stack, you can't assign to it anymore 
once it has a const member variable, which causes all kinds of annoying 
problems. You're just better off if you never declare structs with const or 
immutable member variables.

- Jonathan M Davis