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Subject: Re: [boost] [fusion] Trouble using BOOST_FUSION_ADAPT_ADT with boost::fusion::operator<
From: Joel de Guzman (joel_at_[hidden])
Date: 2012-01-18 03:16:47

On 1/18/2012 12:33 PM, Nathan Ridge wrote:
>>>>> I'm attempting to adapt a third-party struct into a fusion sequence
>>>>> using BOOST_FUSION_ADAPT_ADT to get access to a nice operator<.
>>>>> Unfortunately compilation fails and I'm not sure if it's a bug or me
>>>>> using the library incorrectly. As best I can read the error message, it
>>>>> looks like the code is attempting to compare adt_attribute_proxy structs
>>>>> instead of the values represented by these. Below is some code that
>>>>> demonstrates the problem. Can anybody help me out?
>>>> You've bumped into a limitation of proxies. Fusion adapted ADTs do
>>>> not have access to the actual L-values of the class members, but only
>>>> through get and set. As such, it actually exposes proxy elements
>>>> (adt_attribute_proxy) instead of the actual elements. The only thing
>>>> you can do with these proxies is either to get the values or set the
>>>> values:
>>> Can't Fusion provide overloads for relational operators that compare
>>> adt_attribute_proxy objects by comparing their values?
>> Yes, that is a reasonable solution. I'd welcome a patch if anyone
>> wants to add such operators. I'm a bit worried about the behavior
>> of such operators, however. It needs to deduce the return types of
>> the actual (non-proxy) overloads. It can be done, but is rather
>> tricky without C++11.
> A slightly different idea: rather than provide relational operators
> for adt_attribute_proxy itself, add support for it to the relational
> opperators of Fusion sequences.
> Now the relational operators will work for ADT sequences themselves.
> The user will still have to call get() explicitly when comparing
> individual elements of the ADT sequences, but that should be rare.
> What do you think of this approach? It does not seem to require
> deducing the type of anything.
> If you're interested I can try this out and submit a patch.

Clever! I like it. We can probably put the customization points
(e.g. call_on_values <but better names please>) in the lower level
"support" module in "extension" namespace. Then the "adapted"
module can simply specialize the CPs for adt_attribute_proxy.

Yeah, nice idea!


Joel de Guzman

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