assignment between array elements using boost multi_array iterator

Question

Working on a Kubuntu 14.04 system with gcc 4.8.4 I ran into the following problem:

Using std:vector, I can assign between vector elements via an iterator:

std::vector<float> v ;
v.push_back(0.0) ;
v.push_back(1.0) ;
auto vv = v.begin() ;
vv[0] = vv[1] ;
assert ( v[0] == v[1] ) ;

Using a boost multi_array, this fails:

typedef boost::multi_array<float, 1> array_type; 
boost::array<array_type::index, 1> shape = {{ 2 }};
array_type a(shape) ;
a[0] = 0.0 ;
a[1] = 1.0 ;
auto aa = a.begin() ;
aa[0] = aa[1] ;
assert ( a[0] == a[1] ) ; // fails, a[0] is unmodified

I can work around this using a different idiom like

aa[0] = *(aa+1) ;

But the code I want to use with the multi_array is written using assignments of the type that doesn't work. What am I missing?

It is 2024 and this is still a bug in Boost.MultiArray. I tried a similar library which doesn't have the bug. godbolt.org/z/4W5o1jG8r — alfC
– alfC, Commented Jan 7, 2024 at 18:15

Kerrek SB · Accepted Answer · 2016-01-04 10:24:49Z

1

The reason is that the iterator involved in operator[] for boost::multi_array is an input iterator, which is not required to be mutable.

edited Jan 4, 2016 at 10:24

Kerrek SB

481k96 gold badges904 silver badges1.1k bronze badges

answered Jan 4, 2016 at 10:07

user5743044

191 bronze badge

Sign up to request clarification or add additional context in comments.

4 Comments

Kerrek SB Over a year ago

Sorry, my comment was complete nonsense! (Though input iterators really don't require op-[] to exist.)

Kay F. Jahnke Over a year ago

Indeed. The boost docu states that 'This is an iterator over the values of A. If NumDims == 1, then it models Random Access Iterator. Otherwise it models Random Access Traversal Iterator, Readable Iterator, Writable Iterator, and Output Iterator.' So in this case it would be a Random Access Iterator.

alfC Over a year ago

The OP code should work. If it didn’t it was a bug in BMA

alfC Over a year ago

You pulled the wrong documentation. mulit_array fulfills the concept of MutableMultiArray which, in this sense is more than a Collection.

Collectives™ on Stack Overflow

assignment between array elements using boost multi_array iterator

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related