[boost] discrepancy between multi_array and its documentation

From: Shawn D. Pautz (sdpautz_at_[hidden])
Date: 2007-11-28 11:53:45

I believe that I have found a discrepancy between multi_array and its 
online documentation.  In the reference manual at 
http://boost.org/libs/multi_array/doc/reference.html both the copy 
constructor and the assignment operator are described as calling the 
copy constructor of the stored type (element).  No mention is made of 
the assignment operator of element.  But in the implementation it 
appears that both functions call element's assignment operator (via 
calls to std::copy or boost::detail::multi_array::copy_n).  Furthermore 
the multi_array assignment operator does not call element's copy 
constructor at all.

Here is a simple program to illustrate the behavior:

 >more ./testMultiArray.cc

#include <boost/multi_array.hpp>
#include <iostream>
using std::cout;
using std::endl;

class Foo
{
 public:
   Foo()
   { cout << "  Foo default constructor" << endl; }
   Foo& operator=(const Foo& foo)
   { cout << "  Foo operator=" << endl; return *this; }
   Foo(const Foo& foo)
   { cout << "  Foo copy constructor" << endl; }
};

int main()
{
   cout << "Calling multi_array extents constructor" << endl;
   boost::multi_array<Foo, 1> myFooArray(boost::extents[2]);

   cout << "\n\nCalling multi_array copy constructor" << endl;
   boost::multi_array<Foo, 1> myFooArray2(myFooArray);

   cout << "\n\nCalling multi_array assignment operator" << endl;
   myFooArray2 = myFooArray;
}

Here are the runtime results (compiled with gcc 3.4.5 against Boost 
1.34.1):

 >./testMultiArray
Calling multi_array extents constructor
 Foo default constructor
 Foo copy constructor
 Foo copy constructor

Calling multi_array copy constructor
 Foo default constructor
 Foo copy constructor
 Foo copy constructor
 Foo operator=
 Foo operator=

Calling multi_array assignment operator
 Foo operator=
 Foo operator=

What are the intended requirements on the stored type?  Does it need to 
be Assignable?  The above example works since Foo is Assignable, but for 
other types (such as nested multi_arrays) the multi_array copy 
constructor will not work.  Could someone either fix multi_array or 
update the documentation?

Shawn Pautz