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digitalmars.D.learn - Inheritance and in-contracts

reply Joseph Rushton Wakeling via Digitalmars-d-learn writes:
Suppose I have a base class where one of the methods has an in-contract, and a 
derived class that overrides it:

/////////////////////////////////////////
import std.stdio;

abstract class Base
{
     abstract void foo(int n)
     in
     {
         assert(n > 5);
     }
     body
     {
         assert(false, "Shouldn't get here");
     }
}

class Deriv : Base
{
     override void foo(int n)
     {
         writeln("n = ", n);
     }
}


void main()
{
     Base b = new Deriv;

     b.foo(7);
     b.foo(3);
}
/////////////////////////////////////////

This outputs,

n = 7
n = 3

In other words, the lack of explicit in-contract on Deriv.foo is being taken as 
an _empty_ in-contract, which is being interpreted as per the rule that a 
derived class can have a less restrictive contract than its base (cf. TDPL 
pp.329-331).

Question: is there any way of indicating that Deriv.foo should inherit the 
in-contract from the base method, without actually calling super.foo ... ?

Following the example on p.331, I did try calling super.__in_contract_format(n) 
(... or this.Base.__in_contract_format(n) or other variants), but that doesn't 
seem to work:

     Error: no property '__in_contract_foo' for type 'incontract.Base'

... so can anyone advise if there is a reasonable way of achieving this?

Thanks,

      -- Joe
Dec 05 2014
next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 12/05/2014 02:39 PM, Joseph Rushton Wakeling via Digitalmars-d-learn 
wrote:


 In other words, the lack of explicit in-contract on Deriv.foo is being
 taken as an _empty_ in-contract, which is being interpreted as per the
 rule that a derived class can have a less restrictive contract than its
 base (cf. TDPL pp.329-331).


This is a known problem with contract inheritance. The following bug 
report mentions the ugly hack of defining assert(0) as the derived's 
'in' contract:

   https://issues.dlang.org/show_bug.cgi?id=6856

Ali
Dec 05 2014
parent Joseph Rushton Wakeling via Digitalmars-d-learn writes:
On 05/12/14 23:45, Ali Çehreli via Digitalmars-d-learn wrote:

 This is a known problem with contract inheritance. The following bug report
 mentions the ugly hack of defining assert(0) as the derived's 'in' contract:

    https://issues.dlang.org/show_bug.cgi?id=6856


Thanks for the clarification.  This is a not-nice situation; FWIW I would
second 
Don's proposal that the absence of an explicit in-contract on the derived-class 
method ought to indicate inheritance of the base contract.

I guess the assert(false) method will do, but I find it as ugly as you do :-(

One further annoyance, pointed out to me by a colleague earlier today: given 
that base and derived in-contracts basically come down to,

    try
    {
        Base.in()
    }
    catch (Throwable)
    {
       Derived.in()
    }

... aren't there some nasty consequences here for memory allocation and the 
generation of garbage?
Dec 05 2014
prev sibling next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Joseph Rushton Wakeling:


 Suppose I have a base class where one of the methods has an 
 in-contract,


It's named "precondition" or "pre-condition".



 Following the example on p.331, I did try calling 
 super.__in_contract_format(n) (... or 
 this.Base.__in_contract_format(n) or other variants), but that 
 doesn't seem to work:

     Error: no property '__in_contract_foo' for type 
 'incontract.Base'

 ... so can anyone advise if there is a reasonable way of 
 achieving this?


Is this a strong need?

Bye,
bearophile
Dec 05 2014
parent Joseph Rushton Wakeling via Digitalmars-d-learn writes:
On 06/12/14 00:24, bearophile via Digitalmars-d-learn wrote:

 Is this a strong need?


Let's put it this way: I don't mind copy-pasting the same in-contract into 
derived class methods.  I'd just rather avoid it, and was hoping there was a
way 
to do so which was trivial.

It's disappointing that the lack of an explicitly empty in-contract doesn't 
imply inheritance of the base contract, but I could live with it much more 
easily if I could explicitly indicate that desired inheritance.
Dec 05 2014
prev sibling parent reply "aldanor" <i.s.smirnov gmail.com> writes:
https://github.com/D-Programming-Language/dmd/pull/4200
Dec 22 2014
next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 12/22/2014 10:06 AM, aldanor wrote:

 https://github.com/D-Programming-Language/dmd/pull/4200


Thank you! This fixes a big problem with the contracts in D.

Ali
Dec 22 2014
parent reply "aldanor" <i.s.smirnov gmail.com> writes:
On Monday, 22 December 2014 at 19:11:13 UTC, Ali Çehreli wrote:

 On 12/22/2014 10:06 AM, aldanor wrote:

 https://github.com/D-Programming-Language/dmd/pull/4200


 Thank you! This fixes a big problem with the contracts in D.

 Ali


It's not my PR but I just thought this thread would be happy to 
know :)
Dec 22 2014
parent Joseph Rushton Wakeling via Digitalmars-d-learn writes:
On 22/12/14 20:12, aldanor via Digitalmars-d-learn wrote:

 On Monday, 22 December 2014 at 19:11:13 UTC, Ali Çehreli wrote:

 On 12/22/2014 10:06 AM, aldanor wrote:

 https://github.com/D-Programming-Language/dmd/pull/4200


 Thank you! This fixes a big problem with the contracts in D.

 Ali


 It's not my PR but I just thought this thread would be happy to know :)


Actually, the author is a friend of mine, and an all-round wonderful guy. :-)
Dec 22 2014
prev sibling parent Joseph Rushton Wakeling via Digitalmars-d-learn writes:
On 22/12/14 19:06, aldanor via Digitalmars-d-learn wrote:

 https://github.com/D-Programming-Language/dmd/pull/4200


Yes, I saw that PR with some joy -- thanks for the link! :-)
Dec 22 2014