C ++类初始化

Question

NOVICE ALERT. I have the following classes

View

class View {
 public:
  explicit View(const TupleSchema& schema)
      : schema_(schema),
        columns_(new Column[schema.attribute_count()]),
        row_count_(0) {
  }
  View(const View& other)
      : schema_(other.schema()),
        columns_(new Column[other.schema().attribute_count()]),
        row_count_(0) {

  }
  explicit View(const View& other, rowcount_t offset, rowcount_t row_count)
      : schema_(other.schema()),
        columns_(new Column[other.schema().attribute_count()]),
        row_count_(0) {

  }
     View(const Column& column, rowcount_t row_count)
      : schema_(TupleSchema::Singleton(column.attribute().name(),
                                       column.attribute().type(),
                                       column.attribute().nullability())),
        columns_(new Column[1]),
        row_count_(row_count) {

  }

 private:
  const TupleSchema schema_;
  scoped_array<Column> columns_;
  rowcount_t row_count_;
};

Block

class Block {
 public:
  Block(const TupleSchema& schema, BufferAllocator* allocator)
      : allocator_(allocator),
        columns_(new OwnedColumn[schema.attribute_count()]),
        view_(schema) {

    }
  }
}
 private:
  BufferAllocator* const allocator_;
  scoped_array<OwnedColumn> columns_;
  View view_;  // Full view on the entire block.
  DISALLOW_COPY_AND_ASSIGN(Block);
};

View Copier

class ViewCopier : public BaseViewCopier {
 public:
  ViewCopier(const TupleSchema& schema, bool deep_copy);
  ViewCopier(const BoundSingleSourceProjector* projector, bool deep_copy);
};

When i use the above as members in another class and i write a constructor for it like below

class SegmentedTable : public BasicOperation {
public:
    SegmentedTable::SegmentedTable(const std::vector<TupleSchema> vp_schemas, BufferAllocator* buffer_allocator)
      : BasicOperation(),
        view_copier_(NULL, NULL) { }
private:
    scoped_ptr<Block> block_;
    View view_;
    ViewCopier view_copier_;
}

i get an error message that no method View::View() is defined. I understand it is because there is no View() constructor of class which is needed because it gets automatically initialized in the SegmentedTable Constructor's initializer list. However i have 2 questions

1) why isn't the same needed for Block.

2) why can i initialize ViewCopier with ViewCopier(NULL, NULL) while i can't do that for View. Doing View(NULL) also tells me no method View::View(NULL) is defined.

I know i have not provided some other class definitions that are used in the example, but i am hoping the question can be answered without them.

Answer 1

It is because when you write:

View view_;

It doesn't create a empty reference like in java, it actually tries to contruct View.

So you should either use a pointer and instanciate it later, or constructing it passing the required parameters.

Or add a constructor to View that doesn't take any parameters.

Same thing for ViewCopier

Answer 2

"However why isn't the same needed for Block."

Because in the initializer list in Block 's constructor, you call the TupleSchema constructor for view_ :

view_(schema) {

"Also why can i intialize ViewCopier with ViewCopier(NULL, NULL) "

Because NULL is implicitly convertible to bool . You are actually calling this constructor:

ViewCopier(const BoundSingleSourceProjector* projector, bool deep_copy);

To prevent this from happening, you can use C++11's nullptr instead of NULL. If you tried this:

ViewCopier(nullptr,nullptr)

you would get a compiler error. That's because nullptr won't implicitly convert to bool .

Answer 3

• Since you declared a pointer to a Block, at no point you tried to construct one (block_ will be initialized to nullptr), so the compiler won't complain about a missing constructor.

• ViewCopier(NULL, NULL) will call ViewCopier(const BoundSingleSourceProjector* projector, bool deep_copy); ; the second parameter is cast to a bool . View has no constructor that takes a pointer; only references.

You can't pass NULL to a constructor or a function that takes a reference; you have to pass something.