Question about pure functions

[anonymous]

On Monday, 16 September 2013 at 07:01:52 UTC, Bienlein wrote:
> Hello,
>
> ich habe a pure method bar() in a class Foo:
>
> class Foo {
> 	int i = 0;
> 	
> 	void bar() pure {
> 	    i++;
> 	}	
> }
>
> main() {
> 	auto foo = new Foo();
> 	foo.bar()	
> }
>
> The pure method bar changes the inst var i. Nevertheless, the
> code above compiles and runs which I find confusing. I assumed
> that changing an inst var by a pure function is considered
> creating a side efect. But the compiler has no problems with 
> this.
>
> Am I getting something wrong here? Thanks for any hints.
> Regards, Bienlein

(Weak) pure functions are allowed to mutate their arguments. 
Methods take the object via a hidden parameter, so that's an 
argument, too.

Mark all parameters const to get a strong pure function. For 
"this" const goes on the method:

class Foo {
	int i = 0;
	
	void bar() const pure {
	    // can't mutate i here
	}
}

See also: http://dlang.org/function.html#pure-functions