2d array from boost::multi_array - unable to compile

Question

I am trying to create a 2d array class based on boost::multi_array. I face two issues in the code given below. (1) The code for the member function col() does not compile saying that ::type’ has not been declared. Where am I going wrong? (2) Is it possible to define the member function data() outside the class? My attempt gives compile error as the typedefs are not available. But I am unable to define the typedefs outside the class because the typedefs in turn require the type T which is available only inside the template class. Thanks.

#include <boost/multi_array.hpp>
#include <algorithm>

template <class T>
class Array2d{
public:
    typedef typename boost::multi_array<T,2> array_type;
    typedef typename array_type::element element;
    typedef boost::multi_array_types::index_range range;

    //is it possible to define this function outside the class?
    Array2d(uint rows, uint cols);
    element * data(){return array.data();}

    //this function does not compile
    template<class Itr>
    void col(int x, Itr itr){
        //copies column x to the given container - the line below DOES NOT COMPILE
         array_type::array_view<1>::type myview  = array[boost::indices[range()][x]];
         std::copy(myview.begin(),myview.end(),itr);
    }

private:
    array_type array;

    uint rows;
    uint cols;
};

template <class T>
Array2d<T>::Array2d(uint _rows, uint _cols):rows(_rows),cols(_cols){
    array.resize(boost::extents[rows][cols]);
}

Armen Tsirunyan · Accepted Answer · 2011-09-26 18:32:14Z

2

array_type::array_view<1>::type

You need template and typename here :)

typename array_type::template array_view<1>::type
^^^^^^^^             ^^^^^^^^

The keyword template is required because otherwise the < and > will be treated as less and greater because the array_type is a dependent name, and therefore whether array_view is a nested template or not is not known until instantiation.

answered Sep 26, 2011 at 18:32

Armen Tsirunyan

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2 Comments

suresh Over a year ago

I had figured out with the help of compiler error message and the input from K-ballo. But why the template requirement?

ildjarn Over a year ago

@suresh : For more details, see this FAQ: What is the ->template, .template and ::template syntax about?

K-ballo · Accepted Answer · 2011-09-26 18:17:46Z

1

(1) The code for the member function col() does not compile saying that ::type’ has not been declared.

array_type is a dependant type on T and array_type::array_view<1>::type is still dependant on T, you need a typename.

(2) Is it possible to define the member function data() outside the class?

It sure is, but it shouldn't be a problem for it to be defined inside the class.

 template< typename T >
 typename Array2d< T >::element* Array2d< T >::data(){ ... }

answered Sep 26, 2011 at 18:17

K-ballo

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2 Comments

suresh Over a year ago

Thanks. I modified the problem line as typename array_type::template array_view<1>::type myview = array[boost::indices[range()][x]]; and it compiled. But then, why this explicit mention of template is required? The compiler suggested it though. (gcc 4.4.3)

K-ballo Over a year ago

@suresh: Because is a dependent template; dependant types need typename and dependent templates need template keywords.

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