我注意到在有效的STL
向量是这样一种序列 默认情况下应该使用。
这是什么意思?似乎忽略效率向量可以做任何事情。
谁能给我一个场景,其中向量不是一个可行的选择,但必须使用列表?
我注意到在有效的STL
向量是这样一种序列 默认情况下应该使用。
这是什么意思?似乎忽略效率向量可以做任何事情。
谁能给我一个场景,其中向量不是一个可行的选择,但必须使用列表?
当前回答
当你在序列中间有很多插入或删除时。例如,内存管理器。
其他回答
当你在序列中间有很多插入或删除时。例如,内存管理器。
这里的大多数答案都遗漏了一个重要的细节:为什么?
你想在集装箱里放些什么?
如果它是int型的集合,那么std::list在任何情况下都将丢失,不管你是否可以重新分配,你只从前面删除,等等。遍历列表的速度较慢,每次插入都需要与分配器进行交互。要准备一个例子是极其困难的,其中list<int>击败vector<int>。即使这样,deque<int>可能更好或关闭,而不仅仅是使用列表,这将有更大的内存开销。
然而,如果您正在处理大而丑陋的数据块——而且数量很少——您不想在插入时过度分配,而由于重新分配而进行复制将是一场灾难——那么您可能,也许,使用list<UglyBlob>比使用vector<UglyBlob>更好。
不过,如果你切换到vector<UglyBlob*>或甚至vector<shared_ptr<UglyBlob> >, - list仍然会滞后。
因此,访问模式、目标元素数量等仍然会影响比较,但在我看来,元素大小、复制成本等。
在向量和列表的情况下,对我来说,主要的区别如下:
向量
A vector stores its elements in contiguous memory. Therefore, random access is possible inside a vector which means that accessing an element of a vector is very fast because we can simply multiply the base address with the item index to access that element. In fact, it takes only O(1) or constant time for this purpose. Since a vector basically wraps an array, every time you insert an element into the vector (dynamic array), it has to resize itself by finding a new contiguous block of memory to accommodate the new elements which is time-costly. It does not consume extra memory to store any pointers to other elements within it.
list
A list stores its elements in non-contiguous memory. Therefore, random access is not possible inside a list which means that to access its elements we have to use the pointers and traverse the list which is slower relative to vector. This takes O(n) or linear time which is slower than O(1). Since a list uses non-contiguous memory, the time taken to insert an element inside a list is a lot more efficient than in the case of its vector counterpart because reallocation of memory is avoided. It consumes extra memory to store pointers to the element before and after a particular element.
因此,记住这些区别,我们通常会考虑内存、频繁的随机访问和插入来决定给定场景中向量和列表的胜者。
当你想在容器之间移动对象时,你可以使用list::splice。
例如,一个图划分算法可以将常数数量的对象递归地划分到数量不断增加的容器中。对象应该初始化一次,并始终保持在内存中的相同位置。通过重新链接来重新排列比重新分配要快得多。
Edit: as libraries prepare to implement C++0x, the general case of splicing a subsequence into a list is becoming linear complexity with the length of the sequence. This is because splice (now) needs to iterate over the sequence to count the number of elements in it. (Because the list needs to record its size.) Simply counting and re-linking the list is still faster than any alternative, and splicing an entire list or a single element are special cases with constant complexity. But, if you have long sequences to splice, you might have to dig around for a better, old-fashioned, non-compliant container.
保留迭代器的有效性是使用列表的原因之一。另一种情况是当你不希望在推送物品时重新分配向量。这可以通过巧妙地使用reserve()来管理,但在某些情况下,仅使用列表可能更容易或更可行。