Re: [boost] Networking TS + Beast, NEW Tutorials, Read this to learn std::net !!!

15 Mar 2019

      Hi,

Am 15.03.2019 01:19, schrieb Vinnie Falco via Boost:
...
On Thu, Mar 14, 2019 at 5:09 PM Ivan Matek  wrote:
...
Quite a weird pattern, where buffer does not have member begin, end.
Maybe a sentence or two describing the rationale and complexity would 
be nice.
I know it is O(1), but it is weird that begin/end are called on 
pointer.
Yeah, well... that's a sore subject. First of all I did not invent
these buffer concepts, they are part of Asio and Networking TS. And
they *used* to have begin() and end(), but then the C++ Standards
Committee decided that they didn't like `const_buffers_1` and
`mutable_buffers_1`, so they got rid of them in the TS draft. Then
they realized that having `const_buffer::begin()` would be confusing.
Does it refer to the beginning of the buffer sequence, or does it
refer to the first byte of that buffer? They decided to get rid of
begin/end entirely for the buffer types and add buffer_sequence_begin
and buffer_sequence_end in its place. And this is why we can't have
nice things.
Don't be sad :-)

template <typename BufferSequenceType>
class buffer_sequence_view {
     private:
         BufferSequenceType& sequence_;
     public:
         buffer_sequence_view(BufferSequenceType& s)       : sequence_{s} 
               {};
         buffer_sequence_view(buffer_sequence_view& other) : 
sequence_{other.sequence_} {};
         // ...

         auto begin() { return buffer_sequence_begin(sequence_); };
         auto end()   { return buffer_sequence_end(sequence_);   };
        // ...
};

template <typename BufferType>
class buffer_view {
     private:
         BufferType& buffer_;
     public:
         buffer_view(BufferType& b)      : buffer_{b}             {};
         buffer_view(buffer_view& other) : buffer_{other.buffer_} {};
         // ...

         auto begin() { return range::begin(buffer_.data()); };
         auto end()   { return range::end(buffer_.data());   };
        // ...
};

for( const auto& single_buffer : buffer_sequence_view(buffer_sequence) ) 
{
     // iterates over all elements of the buffer sequence ...
}

for( const auto& c : buffer_view(buffer) | view::take(42) ) {
     // iterates over the first 42 characters of a buffer (sequence) ...
     // I hope my interpretation of range::begin() and range::end() is 
correct;
     // this is completely untested. ;-)
}

So you can still have all the nice things with ranges based for loops, 
C++20 ranges, etc. Though I agree, that these views should be part of 
the standard. Otherwise everyone is going to implement them on their own 
with varying quality (you probably already have spotted some issues with 
my untested example code here), or even worse, works without them.

Christof

Re: [boost] Networking TS + Beast, NEW Tutorials, Read this to learn std::net !!!

Christof Donat