Re: [boost] [iterator adaptor] requires default constructor?

Subject: Re: [boost] [iterator adaptor] requires default constructor?
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-06-11 18:49:38

On 6/11/2010 3:38 PM, Neal Becker wrote:
> It seems an iterator requires a default constructor in order to be adapted
> by iterator adaptor.
>
> This is due to (iterator_adaptor.hpp):279
>
>        iterator_adaptor() {}
>
> Is this really necessary?

I believe if it never gets (explicitly or implicitly) instantiated, 
you're okay.  E.g., on MSVC9, the following compiles, even though X< 
int& > isn't default constructible.

#include <boost/call_traits.hpp>

template< class T >
struct X
{
     X() { }
     X(typename boost::call_traits<T>::param_type x)
         : m_x(x)
     { }
     T m_x;
};

int main(int argc, char* argv[])
{
     X<int> x;
     X< int& > y(x.m_x);
     return 0;
}

However, an explicit instantiation of X< int& > *will* generate a 
compiler error, but I imagine explicit instantiations of iterators are 
infrequent at best.

All of the STL containers (AFAIK) declare a default constructor as 
iterator_adaptor does, regardless of their Allocator template parameter, 
even if the underlying allocator isn't default constructible, so it 
seems to be a rather pervasive situation.

- Jeff