《STL源码剖析》——Vector
vector
vector的源码分为多个文件：vector、<<stl_vector.h>>
vector的底层实现是在<<stl_vector.h>>中
1 vector的基本架构
vector的基本架构如下图所示：
2 vector内的成员变量
在vector本身中，有三个成员变量：_M_start、_M_finish和_M_end_of_storage
_M_start对应的是vector的begin()（目前使用空间的头）
_M_finish对应的是vector的end()（目前使用空间的尾）
_M_end_of_storage对应的则是当前vector的容量（目前可用空间的尾，注意与_M_finish的区别）
3 vector的iterator
vector的iterator是__gnu_cxx::__normal_iterator<pointer, vector>类型的，__normal_iterator的主要声明如下：
template<typename _Iterator, typename _Container>
class __normal_iterator {
protected:
_Iterator _M_current;
typedef iterator_traits<_Iterator> __traits_type;
public:
typedef _Iterator iterator_type;
typedef typename __traits_type::iterator_category iterator_category;
typedef typename __traits_type::value_type value_type;
typedef typename __traits_type::difference_type difference_type;
typedef typename __traits_type::reference reference;
typedef typename __traits_type::pointer pointer;
// ...
}
4 push_back与扩容
重点观察一下vector的push_back方法，其源码如下：
void
push_back(const value_type& __x) {
if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage) {
_GLIBCXX_ASAN_ANNOTATE_GROW(1);
_Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish, __x);
++this->_M_impl._M_finish;
_GLIBCXX_ASAN_ANNOTATE_GREW(1);
} else _M_realloc_insert(end(), __x);
}
push_back函数分为以下步骤：
- 先检查当前是否还存在可用空间，即_M_finish是否等于_M_end_of_storage，如果成立，则跳到2，不成立跳到3
- 构造一个新的元素并插入vector中，并调整_M_finish指针
- 调用_M_realloc_insert，另寻他处，重新开辟空间，将原来的vector中的元素都拷贝到新的vector中，并将新元素也插入vector中，并销毁原来的vector以及其中的元素（步骤3开销很大）
注意：如果对vector的任何操作，导致了vector的重新分配行为，那么原有的iterator都会全部失效
5 insert与扩容
vector提供了很多种的insert，以其中一个为例：在指定的iterator之前插入n个初值为x的元素形式的insert
insert是可能触发vector的重新分配行为的
例子中insert的源码如下：
template<typename _Tp, typename _Alloc>
void
vector<_Tp, _Alloc>::
_M_fill_insert(iterator __position, size_type __n, const value_type& __x) {
if (__n != 0) {
if (size_type(this->_M_impl._M_end_of_storage - this->_M_impl._M_finish) >= __n) {
#if __cplusplus < 201103L
value_type __x_copy = __x;
#else
_Temporary_value __tmp(this, __x);
value_type& __x_copy = __tmp._M_val();
#endif
const size_type __elems_after = end() - __position;
pointer __old_finish(this->_M_impl._M_finish);
if (__elems_after > __n) {
_GLIBCXX_ASAN_ANNOTATE_GROW(__n);
std::__uninitialized_move_a(this->_M_impl._M_finish - __n, this->_M_impl._M_finish, this->_M_impl._M_finish, _M_get_Tp_allocator());
this->_M_impl._M_finish += __n;
_GLIBCXX_ASAN_ANNOTATE_GREW(__n);
_GLIBCXX_MOVE_BACKWARD3(__position.base(), __old_finish - __n, __old_finish);
std::fill(__position.base(), __position.base() + __n, __x_copy);
} else {
_GLIBCXX_ASAN_ANNOTATE_GROW(__n);
this->_M_impl._M_finish =
std::__uninitialized_fill_n_a(this->_M_impl._M_finish, __n - __elems_after, __x_copy, _M_get_Tp_allocator());
_GLIBCXX_ASAN_ANNOTATE_GREW(__n - __elems_after);
std::__uninitialized_move_a(__position.base(), __old_finish, this->_M_impl._M_finish, _M_get_Tp_allocator());
this->_M_impl._M_finish += __elems_after;
_GLIBCXX_ASAN_ANNOTATE_GREW(__elems_after);
std::fill(__position.base(), __old_finish, __x_copy);
}
} else {
const size_type __len = _M_check_len(__n, "vector::_M_fill_insert");
const size_type __elems_before = __position - begin();
pointer __new_start(this->_M_allocate(__len));
pointer __new_finish(__new_start);
__try {
// See _M_realloc_insert above.
std::__uninitialized_fill_n_a(__new_start + __elems_before, __n, __x, _M_get_Tp_allocator());
__new_finish = pointer();
__new_finish = std::__uninitialized_move_if_noexcept_a(this->_M_impl._M_start, __position.base(), __new_start, _M_get_Tp_allocator());
__new_finish += __n;
__new_finish = std::__uninitialized_move_if_noexcept_a(__position.base(), this->_M_impl._M_finish, __new_finish, _M_get_Tp_allocator());
} __catch(...) {
if (!__new_finish)
std::_Destroy(__new_start + __elems_before, __new_start + __elems_before + __n, _M_get_Tp_allocator());
else
std::_Destroy(__new_start, __new_finish,
_M_get_Tp_allocator());
_M_deallocate(__new_start, __len);
__throw_exception_again;
}
std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
_M_get_Tp_allocator());
_GLIBCXX_ASAN_ANNOTATE_REINIT;
_M_deallocate(this->_M_impl._M_start,
this->_M_impl._M_end_of_storage
- this->_M_impl._M_start);
this->_M_impl._M_start = __new_start;
this->_M_impl._M_finish = __new_finish;
this->_M_impl._M_end_of_storage = __new_start + __len;
}
}
}
这个重载的insert源码比较长，但是大致思路如下：
-
if (__n != 0) 检查插入的元素个数n是否为0
-
if (size_type(this->_M_impl._M_end_of_storage - this->_M_impl._M_finish) >= __n) 检查可用空间是否大于等于n，如果此条成立还需要区分新插入的元素个素n与安插点之后的元素个数__elems_after的大小情况
2.1 n < __elems_after
2.2 n ≥ __elems_after
-
如果第二条不成立，那么就需要对vector进行扩容，那么
新vector的长度取决于max(旧长度 x 2，(旧长度 + n个新增元素) x 2)需要注意的是，新的vector不必是在旧的vector上的扩展，图中画在一起，只是为了方便
