C / C++ / MFC

Is it "ok" to get array length this way

2-Jun-09 5:25

There are so many ways...here's one

// Define your class ABC as before - make the DO method <big>public</big>

void MyThreadProc(void* objectAsVoidPtr)
{
  ABC* object = (ABC*)objectAsVoidPtr;
  object->DO();
}

void main()
{
  ABC Object[20];
  for(int i=0;i<20;i++)
    _beginthread(MyThreadProc, 0, (LPVOID)Object+i); //So 20 Threads are created here.
}

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

FocusedWolf1-Jun-09 12:46

1-Jun-09 12:46

#include iostream; //can't paste the << >> things here
#include string;

int main()
{

//works, from what i can see, because you cant compile "int someArray[] = {}" or "int arr[0];" because you cant allocate a 0-length array according to vs.net errors. So your guarenteed atleast 1 item in the array for "sizeof(_array[0])" to always find an item. this is the alternative to doing sizeof(int) or passing the type of the array to the macro.

#define ARRAY_LENGTH(_array) (sizeof(_array)/sizeof(_array[0]))

int someArray[] = {1, 2, 3, 4};

for(int i = 0; i < ARRAY_LENGTH(someArray); i++)
{
std::cout<<*(someArray + i)<

norish1-Jun-09 13:27

norish

1-Jun-09 13:27

ARRAY_LENGTH macro is always ok for real array but not for pointer.

By the way, ARRAYSIZE() macro is predefined in windows.h of some newer sdk and it looks the same form as above. Smile | :)

FocusedWolf1-Jun-09 16:07

1-Jun-09 16:07

wow norish i had no idea about that macro :P when i read that sizeof returned the number of bytes used by an array, i figured if i just divided by the size of the datatype used that i could get the length.

Stuart Dootson1-Jun-09 14:43

1-Jun-09 14:43

If you want a type-safe mechanism for getting the number of elements in an array, you might consider this:

#include <iostream>

template<class A, size_t N>
inline size_t ArraySize(const A ( & )[N]) { return N; }

// Determine array size from an array type - 
//  code from <a href="http://heifner.blogspot.com/2008/04/c-array-size-determination-part-2.html">http://heifner.blogspot.com/2008/04/c-array-size-determination-part-2.html</a>
template <typename T>
struct array_info
{
};

template <typename T, size_t N>
struct array_info<T[N]>
{
   typedef T type;
   enum { size = N };
};

int main()
{
   typedef int (MyArray)[120];
   int a[120];
   float b[120];
   std::cout << ArraySize(a) << std::endl;
   std::cout << ArraySize(b) << std::endl;
   std::cout << array_info<MyArray>::size << std::endl;
}

Here you have mechanisms for getting the number of elements in an array variable (function ArraySize) or an array type (struct array_info). And (unlike the macro version) these two won't compile if given a pointer variable/type - the power of pattern matching sees to that Smile | :)

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

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C^hris L^osinger1-Jun-09 15:35

C^hris L^osinger

1-Jun-09 15:35

or... std::vector

FocusedWolf1-Jun-09 16:15

1-Jun-09 16:15

hmm vectors? i suppose. Is it possible that arrays are more efficient?

Now this part of the code is really sexy:

<br />
template<class A, size_t N><br />
inline size_t ArraySize(const A ( & )[N]) <br />
{ <br />
	return N; <br />
}<br />
<br />
int main()<br />
{<br />
	int a[120];<br />
<br />
	std::cout << ArraySize(a) << std::endl;<br />
}<br />

i'm not sure how it works but my guess is that somehow it's magically reading the size from the declaration of the array.

Stuart Dootson1-Jun-09 20:27

1-Jun-09 20:27

FocusedWolf wrote:
possible that arrays are more efficient

Nope - in VS2008, vectors are guaranteed to use contiguous storage (i.e. same as arrays).

FocusedWolf wrote:
i'm not sure how it works but my guess is that somehow it's magically reading the size from the declaration of the array.

That's the one. You pass an array and the compiler instantiates the function with the template parameters appropriate for that array.

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

FocusedWolf2-Jun-09 4:27

2-Jun-09 4:27

Thanks for that info.

Stuart Dootson1-Jun-09 20:32

1-Jun-09 20:32

Not always necessarily better - not until C++ 0x when vectors get array style initialisers (IIU&RC).

Say I have an encryption key that (foolishly) I'm hardcoding in the software - which is simpler:

[edit]Or a better example - say I'm implementing a CRC32, which can be highly optimised by using a static array of byte values[/edit]

const BYTE key[] = { 12, 34, 4, 5, 36, 23, 765, 34 };

std::vector<BYTE> vKey;
vKey.push_back(12);
  :           :
vKey.push_back(34);

Yes, I know Boost has a library[^] that almost gives vectors initialiser syntax...but sometimes you just can't be bothered Smile | :)

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

David Crow1-Jun-09 17:08

David Crow

1-Jun-09 17:08

FocusedWolf wrote:
#include iostream; //can't paste the << >> things here
#include string;

Why not?

#include <iostream>
#include <string>

Did you use the < button above the smileys?

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FocusedWolf2-Jun-09 4:26

2-Jun-09 4:26

Hmm:

#include <iostream>
#include <string>

O i see... if this works then the lt,gt things are auto replacing my < >. Ya didn't realize that yesterday lol