I'm moving a fairly large project over to C++0x, since it is about to be published (crosses fingers) and we need to stay up with the times. In this project, we use the wonderful boost::assign::list_of to assign collections (since initializer lists don't exist yet). We are using g++ 4.5.1 and Boost 1.46.1.
When switching to C++0x, bits of code like this break:
std::vector<int> vec;
vec = boost::assign::list_of(1)(2)(3);
with the following error:
break_list_of.cpp:7:41: error: ambiguous overload for ‘operator=’ in ‘vec = ((boost::assign_detail::generic_list<int>*)boost::assign::list_of(const T&) [with T = int]().boost::assign_detail::generic_list<T>::operator() [with T = int, boost::assign_detail::generic_list<T> = boost::assign_detail::generic_list<int>, boost::assign_detail::generic_list<T>::Ty = int](((const boost::assign_detail::generic_list<int>::Ty&)((const boost::assign_detail::generic_list<int>::Ty*)(&2)))))->boost::assign_detail::generic_list<T>::operator() [with T = int, boost::assign_detail::generic_list<T> = boost::assign_detail::generic_list<int>, boost::assign_detail::generic_list<T>::Ty = int](((const boost::assign_detail::generic_list<int>::Ty&)((const boost::assign_detail::generic_list<int>::Ty*)(&3))))’
/usr/include/c++/4.5/bits/vector.tcc:156:5: note: candidates are: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = int, _Alloc = std::allocator<int>]
/usr/include/c++/4.5/bits/stl_vector.h:336:7: note: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = int, _Alloc = std::allocator<int>, std::vector<_Tp, _Alloc> = std::vector<int>]
/usr/include/c++/4.5/bits/stl_vector.h:357:7: note: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_CharT>) [with _Tp = int, _Alloc = std::allocator<int>, std::vector<_Tp, _Alloc> = std::vector<int>]
The problem is that there are now multiple ways to assign to vectors (and all the other collection types). Ideally, I would replace all usage of boost::assign::list_of with C++0x initializer lists, but, alas, I am lazy. Furthermore, I think I know how to fix this (see example below) and think the Boost community would benefit from having boost::assign::list_of work in C++0x and C++.
template <typename T>
struct is_initializer_list : boost::false_type { };
template <typename T>
struct is_initializer_list<std::initializer_list<T> > : boost::true_type { };
template <typename T>
struct wrapped_type
{
wrapped_type(const T& val) :
_val(val)
{ }
template <typename TOut,
// use new support for default template arguments in functions
typename TPhony = typename boost::disable_if<is_initializer_list<TOut> >::type
>
operator TOut() const
{
return _val;
}
private:
T _val;
};
I was wondering if anybody has solved this problem (or a similar one) already. I would patch it myself, but I only have g++ and clang to test it on. Also, there is a high probability that my solution is flawed in some way.
Thanks for the help! And, as always, thanks for the wonderful library.
--
- Travis Gockel