Re: How to clean boost::asio::streambuf before writing to it?

From: Gavin Lambert (boost_at_[hidden])
Date: 2020-06-10 08:05:10

On 10/06/2020 18:12, Jupiter wrote:
> Sorry, this is really a silly question, but I could not find a clear
> API in streambuf  reference,

https://www.boost.org/doc/libs/1_73_0/doc/html/boost_asio/reference/streambuf.html

> I have following code to call PacketData
> in a loop, the buffer.get() looked to accumulate previous data in the
> buffer, so I should call buffer.clear() before calling buffer.get(),
> but apparently there no ‘clear’ in boost::asio::basic_streambuf.
> 
> PacketData(const msgpack::sbuffer &sb, boost::asio::streambuf &buffer)) {
> .......
> std::ostream os(buffer.get());
> os.write((char *)&header, sizeof(header));
> os.write((char *)sb.data(), sb.size());
> .......
> }

Firstly, make sure you're using separate streambufs for read and write 
operations -- this is mandatory if you have async reads/writes 
concurrently pending, but is generally a good idea anyway.

commit() [1] and consume() [2] are the methods that move or remove data. 
  Which one you need depends on what you're doing with the buffer.

Both of these require the number of characters written/read.

As the examples show, for reading from a socket (or other I/O object) 
you will typically prepare() [3], read, then commit; whereas for writing 
to a socket you will write, then consume.

Note that some of the ASIO methods that accept a streambuf parameter 
directly will do this for you, so you don't always need to call them 
yourself.

[1]: 
https://www.boost.org/doc/libs/1_73_0/doc/html/boost_asio/reference/basic_streambuf/commit.html
[2]: 
https://www.boost.org/doc/libs/1_73_0/doc/html/boost_asio/reference/basic_streambuf/consume.html
[3]: 
https://www.boost.org/doc/libs/1_73_0/doc/html/boost_asio/reference/basic_streambuf/prepare.html