迭代器失效 [英] Iterator disfunctionality

问题描述

我有一个代码，其中我基本上有正值和负值。负值代表运算符（'+'= - 1，' - '= - 2，'*'= - 3，'/'= - 4）我基本上必须要除以，得到总和等等负值前面的2个数字。

I have a code in which I basically have positive values and negative values. The negative values represent operators('+'=-1,'-'=-2,'*'=-3,'/'=-4) and I basically have to either divide, to make the sum and so on of the 2 numbers preceding the negative value.

std::list<long>::iterator i,j;
for(i=num.begin();++i!=num.end();)
{
            if(*i<0&&num.size()>=2)
            {
                    if(*i==-1)
                    {
                              *i=*--(j=i)+*----(j=i);
                    }
                    else if(*i==-2)
                    {
                              *i=*----(j=i)-*--(j=i);
                    }
                    else if(*i==-3)
                    {
                              *i=*--(j=i)**----(j=i);
                    }
                    else if(*i==-4&&*--(j=i)!=0)
                    {
                              *i=*----(j=i)/(*--(j=i));
                    }//this part is working properly
                    num.erase(--(j=i));
                    num.erase(--(j=i));//here is the only problem
                    break;
    }
}

显然，我试图从中删除一个值不存在的列表。

Apparently, I am trying to erase a value from the list that doesn't exist.

推荐答案

撇开你的一些代码是未定义的行为并且完全不必要地隐藏的事实。

Stepping aside from the fact that some of your code is undefined behavior and completely unnecessarily cryptic.

std::list<long>::iterator i,j;
for(i=num.begin();++i!=num.end();)
{
    // ...
    num.erase(std::prev(i));
    num.erase(std::prev(i));
}

我们知道 num.size（）> = = 2，但我们不知道 i 至少是2开始，所以实际上有两件事需要删除。你的第一次和/或第二次运行循环可能正在尝试擦除不存在的迭代器。

We know num.size() >= 2, but we don't know that i is at least 2 past begin so there are actually two things to erase. It's likely that your first and/or second runs through the loop are trying erase nonexistent iterators.

[edit]显然你的循环检查是 ++ i！= num.end（）。首先，不要这样做。其次，我猜这实际上意味着你开始一个过去的开始，所以这就是你在循环的第一次迭代中第二次擦除失败的原因：

[edit] Apparently your loop check is ++i != num.end(). First, don't do that. Second, I guess that effectively means that you're starting one past begin, so that's why you fail on the 2nd erase in the first iteration of the loop:

[begin] <--> [item] <--> [item] <--> ...
             ^
             i

您正试图删除这两项在 i 前面。只有一个。

You're trying to erase the two items in front of i. There is only one.

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