Effective STL 笔记 -- Item 9: Choose carefully among erasing options
2014年9月03日 00:10
假设有一个容器中存放着 int ，Container<int> c, 现在想从其中删除数值 1963，可以有如下方法：
1: c.erase(remove(c.begin(), c.end(), 1963), c.end()); // c is continguous memory container 2: c.remove(1963); // c is list 3: c.erase(1963) // c is standard associative container
对于 continguous memory container 来说， std::remove() 可以保证返回的 iterator 之前的内存中不包含被删除的值，配合 erase 来用，形成所谓的 erase-remove idiom 。而对于 List 来说，应该调用 list 本身的 remove() 方法。对于 map,set 等等，则必须使用 erase() 方法，而不能使用 remove() ，对 associative container 使用 remove() 可能会覆盖容器存储的内容，甚至破坏容器。
假设我们现在想给定一个条件( bool badValue(int x) )并用这个条件来判断有哪些 Entry 需要删除，对 vector、string、deque、list 可以用下列方法：
1: c.erase(remove_if(c.begin(), c.end(), badValue), c.end()); // for vector, string, deque ... 2: c.remove_if(badValue); // for list.
associative container 没有类似的方法，但可以自行遍历删除：
AssocContainer<int> c; for (AssocContainer<int>::iterator i=c.begin(); i != c.end(); ++i) { if (badValue(*i)) { c.erase(i); //XXX: This is wrong: when this get executed, iterator will become //XXX: invalid, and ++i will lead to undefined behaviour. } } for (AssocContainer<int>::iterator i=c.begin(); i != c.end(); /*Nothing*/) { if (badValue(*i)) { c.erase(i++); //XXX: This works, because we are passing unincreased value to erase, //XXX: but we get it increased before it was passed to erase(). } else { i++; } }
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run in two directions. We can Extract Method to break a long procedure into small and reusable components, and we can Inline Method to get rid of a small method that’s not pulling its weight. We can Pull Up Method to let a bunch of subclasses share common behavior, or we can Push Down Method so only the subclass that uses this behavior needs to know about it. Refactoring then becomes a matter of hill climbing, gradually improving the entire work by making a series of minor changes that facilitate larger and larger changes. When refactoring, how do we know which way is up?