front

  reference       front();
  const_reference front() const;

Returns a reference to the first element in the bitvector.

Precondition: The bitvector is not empty.

Throws nothing.

  reference       back();
  const_reference back() const;

Returns a reference to the last element in the bitvector.

Precondition: The bitvector is not empty.

Throws nothing.

  reference       operator[](size_type n);
  const_reference operator[](size_type n) const;

Returns a reference to the nth element in the bitvector.

Precondition: n < size().

Throws nothing.

  const_reference at(size_type n) const;
  reference       at(size_type n);  

Returns a reference to the nth element in the bitvector.

Throws nothing if n < size(), else throws a std::out_of_range object. If an exception is thrown, there are no effects.