segmentation fault when iterating over empty arrays/objects

[nlohmann]

From an email:

Anyway, a quite serious bug is when one tries to iterate over a empty container (array or object), which results in a segmentation fault due to a 0x0 this pointer. I fixed this by changing the iterator constructors to check if the container is empty, and in that case set the “object_” pointer to a nullptr.

The proposed code fix:

json::const_iterator::const_iterator(const json* j) : object_(j)
{
    if (object_ != nullptr)
    {
        if (object_->type_ == value_type::array)
        {
            if (object_->empty())
// Explicit empty check added
            {
                object_ = nullptr;
            }
            else
            {
                vi_ = new array_t::const_iterator(object_->value_.array->begin());
            }
        }
        else if (object_->type_ == value_type::object)
// *else if* is mandatory since the previous
        {
// block can set pointer to nullptr!
            if (object_->empty())
// Explicit empty check added
            {
                object_ = nullptr;
            }
            else
            {
                oi_ = new object_t::const_iterator(object_->value_.object->begin());
            }
        }
    }
}

[nlohmann]

The bug can be reproduced with the following code:

json j(json::value_type::array);
for (json::iterator it = j.begin(); it != j.end(); ++it) {}