uninitialized_fill与fill的区别

 uninitialized_fill与fill的区别

在STL源码中可以经常看到uninitialized_fill与fill函数.刚开始不明白到底什么时候
用uninitialized_fill还是fill函数.后来模仿写STL源码时,才明白了,这样做一个简单的说明
希望对初学者有所帮助.

先看看两个函数功能:

1.fill是直接对每个元素进行填充value.

2.uninitialized_fill(未初始化填充) 是要根据value的类型来判断使用哪一种方式填充,

如果是POD类型(就是内置类型或普通结构与类(没有指针的成员数据)),就直接调用fill函数.

不是POD类型时,就要遍历每个元素进行构造(调用construct函数).为什么呢?

那是因为*first = value; 类的赋值操作符必须要*first的对象已经生成.

uninitialized_fill一般都是用于未初化填充,也就是说内存区间根本没有对象可言.

此外相关的还有两个: uninitialized_fill_n, uninitialized_copy.原理一样

uninitialized_fill与fill的源码:(模仿sgi stl)

/*uninitialized_fill --作用于未初始化的内存中1.迭代器first指向输出端(欲初始化空间)的起始处2.迭代器last 指向输出端(欲初始化空间)的结束处(前闭后开区间)3.x表示初值*/template <class ForwardIterator, class T>inline void uninitialized_fill(ForwardIterator first, ForwardIterator last, const T& x){ __uninitialized_fill(first, last, x, __value_type(first));//以上,利用__value_type 取出first 的value type;}template <class ForwardIterator, class T, class T1>inline void __uninitialized_fill(ForwardIterator first, ForwardIterator last, const T& x, T1*){typedef typename __type_traits<T1>::is_POD_type is_POD;//就是判断是否是普通类型 __uninitialized_fill_aux(first, last, x, is_POD());}template <class ForwardIterator, class T>inline void __uninitialized_fill_aux(ForwardIterator first, ForwardIterator last, const T& x, __true_type){ fill(first,last, x);//交由高级函数执行,其实就是直接赋值}template <class ForwardIterator, class T>inline void __uninitialized_fill_aux(ForwardIterator first,ForwardIterator last, const T& x, __false_type){ForwardIterator cur = first;__LIN_TRY{for(; cur != last; ++cur)construct(&*cur, x);//需要构造函数}__LIN_UNWIND(destroy(first, cur));}//将[first, last)内的所有元素改填新值abcdtemplate <class ForwardIterator, class T>void fill(ForwardIterator first, ForwardIterator last, const T& value){for(; first != last; ++first)//迭代走过整个区间*first = value;//设定新值}
这是vector中reserve函数
template <class T, class Alloc>void vector<T, Alloc>::reserve(size_t new_size){//determine new minimum length of allcoated storageif(new_size > max_size()){_throwLength();}if(capacity() < new_size){iterator new_start = data_allocator::allocate(new_size);iterator new_finish = new_start;//uninitialized_copy与uninitialized_fill原理一样//这里是要copy到新分配的内存当中.所以要用uninitialized_copynew_finish = uninitialized_copy(begin(), end(), new_start);//析构并释放原vectordestroy(begin(), end());__deallocator();//调整迭代器,指向新vector__start = new_start;__finish = new_finish;__end_of_storage = __start + new_size;}}