Effective STL 笔记: Item 6--Be alert for C++'s most vexing parse
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Effective STL 笔记 -- Item 9: Choose carefully among erasing options

tubo posted @ 2014年9月03日 00:10 in 未分类 , 763 阅读

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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2023年10月05日 07:35

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