Re: [asio-users] ConstBufferSequence requirements document question

[Cheng L. <rhy...@gm...>]

Christopher Kohlhoff wrote:
> On Fri, 25 May 2007 16:53:51 +0800, "Cheng Lian" <rhy...@gm...>
> said:
>> Hi all,
>>
>> I'm writing some Asio specific type requirements checking code to help 
>> constructing a ConstBufferSequence compatible class template. I've read 
>> the following code snippet from the ConstBufferSequence requirements 
>> documentation:
>>
>>      bool equal_buffer(
>>        const X::value_type& v1,
>>        const X::value_type& v2)
>>      {
>>        const_buffer b1(v1);   // (1)
>>        const_buffer b2(v2);   // (2)
>>        return
>>          buffer_cast<const void*>(b1)
>>            == buffer_cast<const void*>(b2)
>>              && buffer_size(b1) == buffer_size(b2);
>>      }
>>
>> The question is about these two lines (1) & (2). In the header file 
>> asio/buffer.hpp I found only one const_buffer constructor that takes 
>> exactly 1 parameter whose prototype is:
>>
>>      const_buffer(const mutable_buffer& b)
> 
> The compiler-generated copy constructor is also a constructor that takes
> exactly one parameter :)

Well, yes, so const_buffer can only be instantiated with a <ptr, size> 
pair or another const_buffer (or an instance of a class that has a 
const_buffer type conversion constructor)?

> 
>> Which implies that v1 should meets the ConvertibleToMutableBuffer 
>> requirements. Once again, from the ConvertibleToMutableBuffer 
>> requirements, I read the following code snippets:
> 
> Obviously you can't have ConvertibleToMutableBuffer as part of the
> requirements for ConstBufferSequence, since only the conversion
> mutable->const is safe, not the other way around. I.e. you shouldn't be
> able to get a mutable buffer out of a constant buffer sequence.
> 

Sorry, I didn't express myself clearly. I means v1 must be initialized 
by an object that meets the ConvertibleToMutableBuffer requirement since 
const_buffer only have a default copy constructor and a constructor that 
accepts a mutable_buffer.

>>      mutable_buffer u(a);
>>      mutable_buffer v(a);
>>      ...
>>
>> But, here's the problem, in asio/buffer.hpp, there's no mutable_buffer 
>> constructor that takes exactly 1 parameter. And my testing code failed 
>> to compile until I made the following two changes:
> 
> What type were you instantiating the test code on when it failed to
> compile? The initialisation of the form:
>

I think I've found out my mistake. At first I used a std::string v1 to 
initialize the following b1. But it's clear that std::string doesn't 
meets the ConvertibleToMutableBuffer requirement.

>   const_buffer b1(v1);
> 
> is exactly what is performed inside asio's implementation, so it should
> work for a type X to meet the necessary ConstBufferSequence
> requirements.

Where exactly is the code that you refer to? I checked the buffers 
example, and in the shared_const_buffer constructor, and in this 
example, you use boost::asio::buffer() rather than the copy constructor 
to initialize the buffer_ member.

> 
> Cheers,
> Chris
> 
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