buffer_cat

[akrzemi1]

The requirement, as per Capy's vision documents:

Buffer sequences should compose at compile time:

auto combined = buffer_cat(header_buffers, body_buffers);
send(combined);  // no allocation, single writev() call

Buffer sequences of different types need to be supported.

The following use case, of concatenating ranges of const_buffer with just const_buffer, should work

void test_case(ConstBufferSequence auto buf)
{
  const_buffer header1 = /*...*/;
  auto slice1 = slice(buf, 0, 50);
  const_buffer header2 = /*...*/;
  auto slice2 = slice(buf, 50, 150);
  ConstBufferSequence auto result = buffer_cat(header1, slice1, header2, slice2);
}

So the result is conceptually analogous to applying ranges::concat to ranges of const_buffer, except that:

1. a raw const_buffer is promoted to a one-element range.
2. empty const_buffer elements can be skipped.

The result of concatenating a ConstBufferSequence with a MutableBufferSequence is a ConstBufferSequence.

Sources:

• Capy's vision: https://github.com/cppalliance/capy/blob/develop/papers/B1004.io-buffers.md#zero-allocation-composition
• Boost.Beast: https://www.boost.org/doc/libs/latest/libs/beast/doc/html/beast/ref/boost__beast__buffers_cat.html