[SOLVED] How can i pass an entire Array into a function parameter by value? (no pointer pass)

NevemTeve · 07-30-2012, 06:48 AM

(Well, most likely you don't have to copy those arrays, but if you are so keen to do so, use either memcpy or nest the arrays into structures; structures can be passed by-value, even if it is a very unusual and un-rational thing to do.)

dwhitney67 · 07-30-2012, 06:58 AM

Quote:

Originally Posted by esgol

But as i repeat, i havent asked councils on this neither referared, JUST HOW TO PASS BY VALUE A DAMNED ARRAY not WHY I SHOULD or IF I SHOULD.
]

In C++:

Code:

#include <array>

static const int ARRAY_SIZE = 10;

void functionCopy(std::array<int, ARRAY_SIZE> a)
{
    // 'a' is a copy of 'arr'
}

void functionSame(std::array<int, ARRAY_SIZE>& a)
{
    'a' is the same as 'arr'
}

int main()
{
    std::array<int, ARRAY_SIZE> arr;

    functionCopy(arr);

    functionSame(arr);
}

P.S. std::array is part of C++0x standards; you may need to specify -std=c++0x when compiling the code above. The alternative is to rely on an std::vector.

pixellany · 07-30-2012, 07:01 AM

Quote:

But as i repeat, i havent asked councils on this neither referared, JUST HOW TO PASS BY VALUE A DAMNED ARRAY not WHY I SHOULD or IF I SHOULD.

I can't help with the substantive issue, but I will share with you a reality of forums like LQ:

You will often not get the answer to your question, but the answer to the question that you should have asked. Corollary: People are reluctant to help others with the details of how to go down the wrong path.

Better to focus on what people are trying to say than to just get upset because noone is directly answering your question.

Sergei Steshenko · 07-30-2012, 10:02 AM

Quote:

Originally Posted by esgol

Yes this is what i ment by conncurent safe. Its the obvious concurency problem.

Yes there are ways to programm concurent with pointers, but since I have only theoritical knowledge of the issue, i'll develop the API as Serial 1st trying to make it as object oriented and functional as possible keeping always in mind how the obgjects and methods, of a Multithread version should be, trying in the same time to secure the atomicity of operations as much as possible avoifing throwing pointers here and there.

If i was sending the arrays pointers then i should use mutexes or make free-lock programming designing the structure of the code entirely different.

For start trying having the code totaly obgect oriented and pure functional its a good start i think :S

But as i repeat, i havent asked councils on this neither referared, JUST HOW TO PASS BY VALUE A DAMNED ARRAY not WHY I SHOULD or IF I SHOULD.

Thats what sergei did not understood thanks for the further interest however, most usually don't even care

P.S.
dwhitney wrote

Lol dwhitney67 ^_^
http://farm4.static.flickr.com/3148/...269be596e0.jpg

Quote:

I have only theoritical knowledge of the issue

- so, it's time to start learning and understanding, isn't it ?

Quote:

Its the obvious concurency problem

- it is not necessarily a problem. For example, do you know what "atomic operation" is, what "atomic read-modify-write" is ?

Regarding the latter, it is the mechanism to acquire locks, and the whole point is that all the contenders are trying to access the same memory location.

Your insistence on passing by value without your explanation why you need shows your apparent misunderstanding of the subject. To make it more clear for you - suppose a human enters some data, say, a "Hello" string.

The human is supposed to be lazy, so he/she will enter the string just once - if you demand from the human to enter the string more than once, the human will most likely look for another program.

So, if you accept that, and if you still think you need multiple copies of the "Hello" string, you need to somehow replicate it. So, why not replicating it in the functions needing to process it ? And then, when you have a copy per function, you don't have the potential concurrency problems because there will be no concurrent access to the same data.

Quote:

If i was sending the arrays pointers then i should use mutexes

- you have so far failed to explain why you need all those complications for read-only data, and you apparently fail to understand you better not use the same data for both input and output, i.e. you better do not process in place. If you follow that principle, you again do not have concurrent access problem.

Regarding read-only data - I suggest first to read http://en.wikipedia.org/wiki/Const-correctness . You'll hopefully be able to find an answer how dealing with data pointed to by pointers avoid writability of the data.

Quote:

the code totaly obgect oriented and pure functional its a good start

- I think you again have a misunderstanding.

Though there is no strict definition of "object oriented", typically when people talk/think about it, classes and their instances are meant, and classes often have data members (with setters and getters for purists). The data members hold state, while functional paradigm gravitates towards pure functions which are stateless.

Sergei Steshenko · 07-30-2012, 10:03 AM

Quote:

Originally Posted by NevemTeve

(Well, most likely you don't have to copy those arrays, but if you are so keen to do so, use either memcpy or nest the arrays into structures; structures can be passed by-value, even if it is a very unusual and un-rational thing to do.)

And we also have unions, don't we ?

NevemTeve · 07-30-2012, 12:58 PM

Very Off: Years ago, working with OpenSsl, I had a happy(?) moment when I realised that between two version of OpenSsl, typedef des_key_schedule has been changed from array to struct.
my code was sg like this:

Code:

myfun (des_key_schedule key) {
/* key is a pointer in the older version
   (call-by-reference) */
/* key is a structure in the newer version
   (call-by-value) */

This little difference did cause me a bit of head-ache, as changes in 'key' never got returned to caller in the newer version.

esgol · 07-31-2012, 01:58 AM