To allow deep copying from an existing GenericValue, an
explicit "copy constructor" (with required Allocator param)
and an "CopyFrom" assignment function are added.
Document d; Document::AllocatorType& a = d.GetAllocator();
Value v1("foo");
// Value v2(v1); // not allowed
Value v2(v1,a); // make a copy
RAPIDJSON_ASSERT(v1.IsString()); // v1 untouched
d.SetArray().PushBack(v1,a).PushBack(v2,a);
RAPIDJSON_ASSERT(v1.Empty() && v2.Empty());
v2.CopyFrom(d,a); // copy whole document
RAPIDJSON_ASSERT(d.IsArray() && d.Size()); // d untouched
v1.SetObject().AddMember( "array", v2, a );
d.PushBack(v1,a);
Additionally, the Handler implementation in GenericDocument is made
private again, restricting access to GenericReader and GenericValue.
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