Parts of std::vector implementation from https://gcc.gnu.org/onlinedocs/gcc-9.2.0/libstdc++/api/a07959.html

stdVector.h

pointer
_M_allocate(size_t __n)
{
 typedef __gnu_cxx::__alloc_traits<_Tp_alloc_type> _Tr;
 return __n != 0 ? _Tr::allocate(_M_impl, __n) : pointer(); //(3)
}

void
_M_create_storage(size_t __n)
{
 this->_M_impl._M_start = this->_M_allocate(__n); //(2)
 this->_M_impl._M_finish = this->_M_impl._M_start;
 this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
}

/**
 *  @brief  Creates a %vector with default constructed elements.
 *  @param  __n  The number of elements to initially create.
 *  @param  __a  An allocator.
 *
 *  This constructor fills the %vector with @a __n default
 *  constructed elements.
 */
explicit
vector(size_type __n, const allocator_type& __a = allocator_type())
: _Base(_S_check_init_len(__n, __a), __a) //(1)
{ _M_default_initialize(__n); }

/**
 *  @brief  Subscript access to the data contained in the %vector.
 *  @param __n The index of the element for which data should be
 *  accessed.
 *  @return  Read/write reference to data.
 *
 *  This operator allows for easy, array-style, data access.
 *  Note that data access with this operator is unchecked and
 *  out_of_range lookups are not defined. (For checked lookups
 *  see at().)
 */
reference
operator[](size_type __n) _GLIBCXX_NOEXCEPT
{
  __glibcxx_requires_subscript(__n);
  return *(this->_M_impl._M_start + __n); //(4)
}