Ian,
Thanks for the quick reply.
On 05/16/2013 09:57 AM, Ian Craggs
wrote:
Hi Frank,
it's been my plan to have a C++
wrapper for the C APIs. If you are able to contribute your code
to this project, that would be great. If not, I'll do my own.
That's the place for proper C++ conformance.
Yes, I would be happy to contribute it. I can start posting code
within the next few weeks and would appreciate input on a number of
arbitrary design decisions.
(1) Make the headers C++ aware internally:
#if defined(__cplusplus)
extern "C" {
#endif
Once the C++ wrapper is written, then C++ programs can use that,
and this would become moot. I have a preference for keeping C++
related stuff out of the C code, but I can see the logic.
This one is pretty trivial, is fully C-conformant, and is fairly
common in C libraries these days. It just helps the C++ programmer
in not having to remember which headers need to be wrapped. It's not
that you're adding any C++ to the header, but rather just making it
safe for a C++ app to use directly.
I think that a number of C++ apps might still use the C library
directly even if there were a C++ wrapper available.
(2) Observe proper const-ness for pointer parameters if there is
no intent in updating the item, particularly "const char*" for
"char*"
DLLExport int MQTTAsync_create(MQTTAsync* handle,
const char* serverURI, const char* clientId, ...
When I've used const in the past, it's only caused pain.
Ah, yes, but the pain of not using it can be so much worse!
"The const and volatile properties are new with the ANSI standard.
The purpose of const is to announce objects that may be placed in
read-only memory, and perhaps to increase opportunities for
optimization."
Which is not the same as the purpose here. In the latest version
of gcc on my machine, using const seems not to cause the problems
I was expecting, but I'd have to check on all the compilers we
use, MS, and old versions of gcc, before adding this change.
In C it is a good programming practice since it helps indicate the
direction of the parameter into or out of the function. But in C++,
it's a downright requirement. To use literal strings you are forced
by the compiler to cast away the const-ness:
MQTTAsync_create(handle, (char*) "tcp://localhost:1883",
(char*) "MyClientID", ...
That's wrong, and pretty ugly too.
Just consider the C library these days. It uses const in this way,
like:
void *memcpy(void *dest, const void *src, size_t n);
This indicates that the contents of 'dest' will be updated, but
'src' will not.
Shouldn't the definition for clientId for example, be "const char
const * clientId"? That is, both the pointer and the contents are
immutable.
Well since the pointer itself it passed by value, you don't need to
do this for a function parameter, just like you wouldn't bother to
write "void do_something(const int n);"
But you would want to add the extra const if you were declaring a
constant string in memory:
Replace:
#define ADDRESS "tcp://swtest.hursley.ibm.com:1883"
with:
const char* const ADDRESS =
"tcp://swtest.hursley.ibm.com:1883";
(3) Use const values instead of #define whenever possible, since
#define has no respect for namespaces, etc.
const int MQTTASYNC_SUCCESS = 0;
instead of
#define MQTTASYNC_SUCCESS 0
See above. I've encountered pain when this before when this
should be simple. This is better done in a C++ header I think.
Well the Paho/MQTT macros are pretty-specifically named, so probably
won't cause a problem. But it must be remembered that #define's will
overwrite anything in other files that include them regardless of
language constraints, like namespaces. So they can get ugly when an
application mixes different 3rd party libraries.
When pulling a C library into a C++ app, if the library uses const
declarations, any name clashes can be eliminated by putting the C
lib into a namespace of it's own:
// my_cpp_app.h
namespace some_c_lib {
#include "some_c_lib.h"
}
But #define's would "jump out" of that namespace.
BTW, I can submit a patch for most of this. I'm just now starting to
wander into MQTT and wanted to throw out the idea. But I don't have
a lot of different compilers to test it out on at the moment
Thanks for your time,
Frank
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