A modern way of doing this

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Hi all I have a really old c++ program that sends a UDP broadcast to my local LAN and returns the result in a char array (this->buf)

// Initial discovery broadcast - should return a request string in a char array.
this->Result = recvfrom(this->udpsocket, this->buf, sizeof(this->buf), 0, (sockaddr *)&this->si_other, &this->slen);

It works perfectly but I'm wondering what would be the *modern* way of doing this in c++ ? by modern I mean using a container other than a raw array of chars for the return value.

Posted 28-Jan-16 1:57am

pkfox

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dan!sh 28-Jan-16 8:09am

Joke begins: Use MVC, responsive UI, web API and json. Everyone is doing it this way. Oh, and don't forget to host it on cloud and access from your refrigerator or watch. It will be as modern as it gets but may not do what you need it to.
Joke ends

OriginalGriff 28-Jan-16 8:19am

If only some people didn't really think like that... :sigh:

pkfox 28-Jan-16 8:36am

Poor man needs help :-)

Mohibur Rashid 28-Jan-16 19:56pm

Well, those who like to think that way are considered 'not good enough'

Jochen Arndt 28-Jan-16 8:30am

You may use a reinterpret_cast instead of the C cast. Nothing else from my point of view.

pkfox 28-Jan-16 8:38am

Ok thanks - I just thought there might be a collection such as c# List to store the return value in.

Jochen Arndt 28-Jan-16 9:35am

recvfrom() is a C style function and requires therefore C parameters.

You receive a block if binary data. The only possible collection would be an array type storing bytes which is pre-allocated to the max. size.

dan!sh 28-Jan-16 22:50pm

Joke 2 (sorry): Modern and LAN in same sentence in 2016? Have you not heard of WiFi?

pkfox 29-Jan-16 10:45am

Why is Wi-Fi not a LAN ? you tell my why it isn't and I'll tell why it is

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Andreas Gieriet · Accepted Answer · 2016-01-28T05:55:00

Solution 2

The C++ Standard newer than C++98 states that vector memory is contiguous.
E.g. see c++ - Is it safe to assume that STL vector storage is always contiguous? - Stack Overflow[^] and vector - C++ Reference[^].
You could do something like this:

C++

class MyClass {
private:
    typedef unisnged char Byte;
    enum { MY_MESSAGE_SIZE = 1024 };
    ...
    std::vector<Byte> buffer;
    ...
public:
    // constructor
    MyClass()
    : ...
    , buffer(MY_MESSAGE_SIZE) // IMPORTANT: This allocates a vector of the given size
    , ...
    { ... }
    
    void doSomethingUseful(...) {
        ...
        result = recvfrom(udpsocket, &(buffer[0]), MY_MESSAGE_SIZE, 0, (sockaddr *)&si_other, &slen);
        ...
    }
};

Cheers
Andi

Posted 28-Jan-16 5:55am

Andreas Gieriet

Comments

CPallini 28-Jan-16 15:58pm

5.

Andreas Gieriet 28-Jan-16 17:11pm

Thanks for your 5!
Cheers
Andi

pkfox 28-Jan-16 18:57pm

Didn't realise I'd given you a five ;-) I'm new to this game - thanks a lot

Andreas Gieriet 28-Jan-16 19:03pm

Thanks for your 5.
Cheers
Andi

Albert Holguin 28-Jan-16 19:39pm

+4.... this really doesn't buy you anything... for that matter, why a std::vector and not a std::array? the socket recvfrom() call isn't going to change size, no point in using a vector.

Andreas Gieriet 28-Jan-16 19:53pm

Depends on the C++ standard you are using.
Before C++11, std::vector is the way to do it.
With C++11 and above, std::array is preferred, I agree.
The member would then be defined as std::array<Byte, MY_MESSAGE_SIZE> buffer, initializing with buffer() and accessing with buffer (ore likewise &(buffer[0])).
Thanks for your hint and your 4!
Cheers
Andi
PS: What it buys you is the "hidden" memory management and the C++ way to access the container by iterators etc. It's a matter of what goal you try to achieve an also a matter of taste, I'd say.

Albert Holguin 28-Jan-16 20:02pm

Except you're completely bypassing iterators by loading data the way you're showing right? You're also bypassing any memory management. In your example, if the user didn't initialize the vector, you'd get a nice crash out of that. So you've given someone a false sense of security.

Andreas Gieriet 28-Jan-16 21:47pm

Let's abstract a bit from the given example and talk about how to interface from C++ to C functions. It is a common pattern to use those containers from C++ which guarantee contiguous memory. This are std::vector for pre-C++11 plus std::array for C++11 and above.
If the client code is C++, I prefer C++ containers.
Since the OP had a similar intent as far as I understand, I gave him the criterion to decide: "contiguous memory container".
In C/C++ you can always do anything - these languages *enable* you to do great things - if you use the languages wisely.
If it is advised to change "running" code without cause is another topic, I'd say.
Cheers
Andi

Albert Holguin · Accepted Answer · 2016-01-28T05:54:00

Solution 1

Why would you change something that works? UDP socket calls are relatively simple, keep it that way...

If anything, I'd make sure that call was asynchronous and give it a timeout period in case you never get a response (otherwise, you just have a call that can hang your program).

Posted 28-Jan-16 5:54am

Albert Holguin

Comments

pkfox 28-Jan-16 18:59pm

Hi there , I'm not going to change it I was just being curious - thanks for your time

Albert Holguin 28-Jan-16 20:12pm

curiosity is a great way to learn... keep it up

Albert Holguin 28-Jan-16 20:17pm

FYI... sizeof() gives you the size of whatever you're passing to it, if you pass an char[] to it, it'll give you the size of the address, not the size of the buffer... looks like you're using it wrong... also, you shouldn't have to specify "this" all the time if you're already within the object, makes the code unnecessarily verbose... if you're making calls within a class, "this" is implied.

pkfox 29-Jan-16 10:48am

Hi Albert what should I use instead of sizeof ?

Albert Holguin 29-Jan-16 14:59pm

The actual size of the array that you're using. If you're using a raw char array, you should know the size since you allocated it... if you're using a container (say vector, even though I don't think it's applicable to use a vector for this) then use whatever size() function the container has.