http::basic_parser::on_body_init

Called once before the body is processed.

void
on_body_init_impl(
    boost::optional< std::uint64_t > const& content_length,
    error_code& ec);

This virtual function is invoked once, before the content body is processed (but after the complete header is received).

Name	Description
`content_length`	A value representing the content length in bytes if the length is known (this can include a zero length). Otherwise, the value will be `boost::none`.
`ec`	An output parameter which the function may set to indicate an error. The error will be clear before this function is invoked.