On 12/27/2009 7:01 PM, anony wrote:

Eric Niebler pravi:

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On 12/27/2009 4:33 PM, anony wrote:

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I have this variable definition:

float vertices[][3] = { {-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1}, {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1} };

I want to scale all elements of vertices, by some constant, for now I do it like this:

BOOST_FOREACH(float& f, boost::make_iterator_range( boost::begin(vertices[0]), boost::end(vertices[7])))

Yikes! You can't do that. The begin and end iterators point into different arrays.

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{ f *= scale; }

But maybe something more beautiful could be done with Boost.Range?

Are you certain I am wrong? The array is multidimensional and contiguous in memory. The boost::begin() and boost::end() just return pointers. I have checked if all the floats scale correctly and they do.

I'm pretty certain. You're straying into undefined behavior by making a range using iterators from different arrays.

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Why not just use BOOST_FOREACH correctly?

BOOST_FOREACH(float (&rgf)[3], vertices) BOOST_FOREACH(float& f, rgf) f *= scale;

Double loop, oops. Not fancy enough.

Are you joking? -- Eric Niebler BoostPro Computing http://www.boostpro.com

What I see in

vertices[][3] = { ... };

is an a array object of array objects. Nowhere in the standard did I ever see multidimensional arrays specifically mentioned. It's in: 5.3.4 New section 5 (which notes: the type of new int[i][10] is int (*)[10]), and 6 8.3.4 Arrays sections 2, 3, 4, 7, and 8 8.5.1 aggregates sections 10 and 11 on array initialization, 13.1 Overloadable Declarations section 3 which notes that Parameter declarations that differ only in a pointer * versus an array [] are equivalent. That is, the array declaration is adjusted to become a

Since the elements of this array are arrays, these arrays must be contiguous in memory. What you're missing, is that it's required in the standard (8.3.4

anony wrote: pointer declaration (8.3.5) 20.5.6.4 Array modifications, Table 39. and on and on in many more places, all consistently saying the for a multidimensional array, all but the final dimensions are pointers. section 6), to consider an array as a pointer to it's first element, so an array of arrays is an array of pointers (well explained in 8.3.4 section 7 and 8). For example in gcc, this code: #include <iostream> int main() { int array[2][2]; std::cout << std::hex << array[0] << std::endl; std::cout << array[1] << std::endl; std::cout << &array[0][0] << std::endl; std::cout << &array[1][0] << std::endl; std::cout << std::dec << sizeof(array[1]) << std::endl; std::cout << sizeof(array[1][1]) << std::endl; } prints out 0xbff3519c 0xbff351a4 0xbff3519c 0xbff351a4 8 4 on one particular run, showing that array[0], and array[1] are just the addresses of their first elements, their elements are 8 byte pointers to 4 byte ints. The pointers are the elements of the array, and as you pointed out, have to be contiguous. There's nothing in the standard that requires that the rows pointed to have to be contiguous, because they aren't the elements of the array, the pointers to them are. So, the contents of the rows have to be contiguous, and the pointers to the rows have to be contiguous, but the rows themselves don't have to be. There's nothing in the standard that requires it. You get away with it with one set of data on one compiler, because it's easy for the compiler with this data to allocate memory for all the rows at once, but nothing guarantees that it will work. Liking it, or wishing that it was the way the spec has it, doesn't help. You're relying on undefined behavior. Patrick

On Sat, Dec 26, 2009 at 10:45 PM, OvermindDL1 wrote:

On Sat, Dec 26, 2009 at 8:33 PM, anony wrote:

...

I have this variable definition:

float vertices[][3] = {  {-1, -1,  1}, {1, -1,  1}, {1, 1,  1}, {-1, 1,  1},  {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1}  };

I want to scale all elements of vertices, by some constant, for now I do it like this:

BOOST_FOREACH(float& f, boost::make_iterator_range(  boost::begin(vertices[0]), boost::end(vertices[7]))) {  f *= scale; }

Are you sure that is correct, you are only multiplying the first 7 floats (out of 8*3=24)...

On Sat, Dec 26, 2009 at 8:33 PM, anony wrote:

...

But maybe something more beautiful could be done with Boost.Range?

Actually you could make it more 'beautiful' (as in shorter code) using fusion+phoenix if it really is a constant sized array like you have.

This is probably not correct, but I am at work right now so cannot check:

using namespace boost::fusion; using namespace boost::phoenix; using boost::array;

array,8> vertices = {  {-1, -1,  1}, {1, -1,  1}, {1, 1,  1}, {-1, 1,  1},  {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1}  };

for_each(vertices,arg0*=val(scale));  // one line of code

Else, if it is not a static size, then yeah, do what Eric said, even then his way is short enough that you may as well use it regardless.

Er, you will need to joint view the sub array parts or something. It is easy to do, I am just a bit brain-dead right now, sleep...

OvermindDL1 pravi:

On Sat, Dec 26, 2009 at 8:33 PM, anony wrote:

...

I have this variable definition:

float vertices[][3] = { {-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1}, {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1} };

I want to scale all elements of vertices, by some constant, for now I do it like this:

BOOST_FOREACH(float& f, boost::make_iterator_range( boost::begin(vertices[0]), boost::end(vertices[7]))) { f *= scale; }

Are you sure that is correct, you are only multiplying the first 7 floats (out of 8*3=24)...

On Sat, Dec 26, 2009 at 8:33 PM, anony wrote:

...

But maybe something more beautiful could be done with Boost.Range?

Actually you could make it more 'beautiful' (as in shorter code) using fusion+phoenix if it really is a constant sized array like you have.

This is probably not correct, but I am at work right now so cannot check:

using namespace boost::fusion; using namespace boost::phoenix; using boost::array;

array,8> vertices = { {-1, -1, 1}, {1, -1, 1}, {1, 1, 1}, {-1, 1, 1}, {-1, -1, -1}, {1, -1, -1}, {1, 1, -1}, {-1, 1, -1} };

for_each(vertices,arg0*=val(scale)); // one line of code

I guess Boost.Lambda would work also in this case :) Good idea.