Re: [boost] [array] how to construct boost::array as class member

From: Sylvain Pion (Sylvain.Pion_at_[hidden])
Date: 2006-08-09 04:31:02

Jeff Flinn wrote:
> Would not the boost.assign library work here?
> 
>>>class A {
>>>   boost::array<int, 3> a;
>>>   A() : a(boost::assign::list_of(1)(2)(3)(4)) {}
>>>};
> 
> I don't think I've tried it this way yet.

It works, but it's not efficient.  It is even incredibly worse that
doing the assignments by hand (boost 1.33.1 is better than 1.32 though).
See my attached test program.  Here is what I get (with g++ 4.1):

                      #default ctor | #copy ctor | #operator= | #dtor
make_array                1        | 3          | 0          | 4
hand assignments          4        | 0          | 3          | 4
assign::list_of 1.32      4        | 11         | 3          | 15
assign::list_of 1.33.1    4        | 7          | 3          | 11

-- 
Sylvain

#include <boost/array.hpp>
#include <boost/assign.hpp>
#include <boost/version.hpp>
#include <iostream>

struct B
{
  B() { std::cout << "default ctor" << std::endl; }
  B(const B&) { std::cout << "copy ctor" << std::endl; }
  B& operator=(const B&) { std::cout << "assign operator" << std::endl; return *this; }
  ~B() { std::cout << "destructor" << std::endl; }
};

template < typename T > inline
boost::array<T, 3>
make_array(const T& b1, const T& b2, const T& b3)
{
  boost::array<T, 3> a = { { b1, b2, b3 } };
  return a;
}

typedef boost::array<B, 3> Array;

struct A {
  Array array;
  A(const B &b1, const B &b2, const B &b3)
    : array(make_array(b1, b2, b3)) {}
  //{ array[0] = b1; array[1] = b2; array[2] = b3; }
    //: array(boost::assign::list_of(b1)(b2)(b3)) {}
    //: array(b1, b2, b3) {} // not [yet] C++
};

int main()
{
  std::cout << "Using Boost version " << BOOST_LIB_VERSION << std::endl;
  B b;
  A a(b, b, b);
}