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Subject: Re: [boost] [type_traits][function_types] Discard param const qualification, bug or feature?
From: Mostafa (mostafa_working_away_at_[hidden])
Date: 2013-10-01 00:45:39

On Mon, 30 Sep 2013 19:54:56 -0700, Gavin Lambert <gavinl_at_[hidden]>

> On 10/1/2013 2:46 PM, Quoth Mostafa:
>> struct SomeUserClass
>> {
>> static void foo(int const x)
>> {
>> SomeCodeGenClass::foo(x);
>> }
>> };
>> SomeCodeGenClass::foo is a mere parameter forwarder, so the goal is to
>> do it as efficiently as possible. It's signature is constructed from
>> SomeUserClass::foo. For correctness, that should be:
>> void SomeCodeGenClass::foo(int const & x)
> This would also be the correct signature for foo(int x) -- because
> they're the same thing.
>> But, function_types<SomeUserClass::foo>::arg1_type resolves to int, so
>> that add_reference'ing will give the following signature for the TMP
>> constructed SomeCodeGenClass::foo
>> void SomeCodeGenClass::foo(int & x)
> A simple add_reference is obviously the wrong thing to be doing then.
> In general, you can take any parameter type T and wrap it as a "const
> T&" (or if you prefer, "T const&") and it will do the right thing,
> unless T was already a reference. But note that you must use a const
> reference -- a non-const reference won't work.

Not in my particular use case. The code I was working is generated by a
mix of PPMP and TMP techniques. The argument x is eventually forwarded to
some user block of code, where the user expects it to be of the same type
as the one he/she specified in the signature of SomeUserClass::foo. So had
the user specified this instead:

struct SomeUserClass
   static void foo(int x)

Then it is expected that x would be mutable in that forwarded-to block of
code. (The fact that it's actually a reference when it arrives there is
immaterial to the user.) Now, I can make a copy of x before the final
stage, but that would partially defeat the purpose of passing by
reference, since a copy is already made at SomeUserClass::foo.

In sum, this particular problem maybe restricted to PPMP/TMP code, and it
maybe that pure TMP code doesn't suffer from this.

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