on Mon Oct 29 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:
On 29/10/12 22:15, Dave Abrahams wrote:
on Mon Oct 29 2012, Agustín K-ballo Bergé <kaballo86-AT-hotmail.com> wrote:
On 29/10/2012 04:29 p.m., Dave Abrahams wrote:
In the abstract, json looks like:
struct null {}; typedef std::map<std::string, value> object; typedef std::vector<value> array;
typedef variant< null, std::string, double, object, array, bool > value;
Keep in mind that instantiating a standard container with an incomplete type will yield undefined behavior, and that `variant` has to instantiate both `object` and `array` to know their size. In my own JSON attempt I have used Boost.Containers, which do guarantee defined behavior when instantiated with incomplete types.
I haven't instantiated anything at that point in the code, though.
According to the code you provided, you do instantiate the std::vector class template with an incomplete type in both the no_wrapper and wrapper cases.
Actually, I think the wrapper case is perfectly legal... and I can prove it. The following compiles: --8<---------------cut here---------------start------------->8--- namespace wrapper { struct value; template <class T, class U> struct mymap : std::map<T,U> { enum { _size = sizeof(T)+sizeof(U) }; // error if T or U are incomplete }; typedef mymap<std::string, value> object; template <class T> struct myvec : std::vector<T> { enum { _size = sizeof(T) }; // error if T or U are incomplete }; typedef myvec<value> array; typedef variant< null, std::string, double, recursive_wrapper<object>, recursive_wrapper<array>, bool
value_base;
struct value : value_base { value() {}; template <class T> value(T const& x) : value_base(x) {} // no move; I'm in C++03-land value& operator=(value const& x) { value_base const& x_ = x; value_base::operator=(x_); return *this; } }; } wrapper::value x; --8<---------------cut here---------------end--------------->8--- -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost