Re: [Boost-users] [fusion] .at<N>() template member in containers?

7 Oct 2010

      On Wed, Oct 6, 2010 at 12:56 PM, alfC  wrote:
...
Hi,
 Excuse my simple minded questions. Sometimes by asking this question
(often with negative answer) I understand more on the design of the
Boost libraries.
Besides possible redundancy, why is that the fusion containers, for
example fusion::vector, doesn't have a template member function to
extract elements?
 fusion::vector stuff(1.2, 3, "hola");
 stuff.at<2>(); //returns "hola", just like at_c<2>(stuff);
Wouldn't it be more consistent with std::vector v, v[2] or v.at(2);
No, because things like std::vector is an implementation class, hence
it can be done in any way they want, however fusion does not have that
luxury, for example, boost::array is defined as a fusion container, so
you can do something like this:

template<typename T>
void printSecondElement( const T &t )
{
  std::cout << *next(begin(t));
}

Now, you can do something like this with it:

fusion::vector f(42,3.14,'X');
printSecondElement(f);

Or you can do this:
boost::array a(1,2,3);
printSecondElement(a);

Or even this:
struct MyType {
  char a;
  float f;
}
BOOST_FUSION_ADAPT_STRUCT(
    MyType,
    (char, a)
    (float, f)
    )
MyType t('r', 3.14);
printSecondElement(t);

And guess what, it prints the second element of each thing, whether a
fusion::vector, a boost::array, or some custom MyType.

Hence, it is impossible to have at be a member function.

To be frank, you should *never* have any function be a member function
unless it absolutely cannot be created externally, you will have
better compile times, better encapsulation, and it prevents you from
doing nice things like the above.

Re: [Boost-users] [fusion] .at<N>() template member in containers?

OvermindDL1