const Propagation

[Matt Soucy via Digitalmars-d]

On 12/29/2014 02:07 PM, anonymous wrote:
> On Monday, 29 December 2014 at 13:20:39 UTC, Julian Kranz wrote:
>> Thank you for your answer. This kind of thing also works for C++, but that would mean that I would implement the whole visitor twice - one const and one non-const version. Is that really the only way? Can't I tell the D compiler that "the argument of that lambda shares the const-ness of the current object"?
>>
>> D offers "inout"; this actually aims into the right directing, but I guess it does not help here.
>>
>> Is there any "static if"-something construct to check the const-ness of an object?
> 
> There's a pattern I suggested before[1] that I'd like to mention
> in addition to the template solutions Steven Schveighoffer and
> Daniel Kozak gave:
> 
> Call the non-const overload from the const overload and cast
> accordingly.
> 
> In your case:
> 
>    void blah(void function(Hugo h) f) {
>      f(this);
>    }
>    void blah(void function(const Hugo h) f) const {
>      (cast(Hugo) this).blah(cast(void function(Hugo)) f);
>    }
> 
> This is safe as long as the non-const overload does not mutate
> the object when f doesn't. BUT you have to make sure of that
> yourself; the compiler can't help anymore.
> 
> [1]
> http://stackoverflow.com/questions/22442031/how-to-make-a-template-function-const-if-the-template-is-true/22442425#22442425
Wait, is there a reason that you cast away const instead of adding it? I haven't done too much with D const lately, but in C++ I know that I would have the non-const version call the const version instead - seems a bit safer, and the compiler can check it a bit beter.

-- 
Matt Soucy
http://msoucy.me/

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