net: add a convenience getter accessor for dns Header to DnsQuery

net/dns/dns_query.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_DNS_DNS_QUERY_H_
 #define NET_DNS_DNS_QUERY_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 
 namespace net {
 
+namespace dns_protocol {
+struct Header;
+}
+
 class IOBufferWithSize;
 
 // Represents on-the-wire DNS query message as an object.
 // TODO(szym): add support for the OPT pseudo-RR (EDNS0/DNSSEC).
 class NET_EXPORT_PRIVATE DnsQuery {
  public:
   // Constructs a query message from |qname| which *MUST* be in a valid
   // DNS name format, and |qtype|. The qclass is set to IN.
   DnsQuery(uint16_t id, const base::StringPiece& qname, uint16_t qtype);
   ~DnsQuery();
@@ skipping 11 matching lines @@
   base::StringPiece question() const;
 
   // IOBuffer accessor to be used for writing out the query.
   IOBufferWithSize* io_buffer() const { return io_buffer_.get(); }
 
   void set_flags(uint16_t flags);
 
  private:
   DnsQuery(const DnsQuery& orig, uint16_t id);
 
+  // Convenience for header access.
+  dns_protocol::Header* header();

[pauljensen, 2015/12/29 12:38:02]

I'm not really a fan of making something hacky (the reinterpret_cast) easier to
use.

[tfarina, 2015/12/29 13:57:43]

dns_response.h already has an accessor like this. I don't consider something
from the C++ cast-system like reinterpret_cast a hack.

[pauljensen, 2015/12/29 15:06:22]

Casting something to a different type and dereferencing is a hack in my book. 
It violates C aliasing rules and optimizing compiler are apt to reinterpret the
code in unforseen ways.

+
   // Size of the DNS name (*NOT* hostname) we are trying to resolve; used
   // to calculate offsets.
   size_t qname_size_;
 
   // Contains query bytes to be consumed by higher level Write() call.
   scoped_refptr<IOBufferWithSize> io_buffer_;
 
   DISALLOW_COPY_AND_ASSIGN(DnsQuery);
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_DNS_QUERY_H_