我应该避免“goto”在这样的情况下？ [英] Should I avoid "goto" in situations like this?

问题描述

我正在创建一个连接迭代器，即迭代器将遍历 int s在 int ** 。

I was making a "concatenating iterator", i.e. an iterator that would iterate over the ints in an int**.

其构造函数需要：

• 数组 $
$ ，表示每个子数组的开头。 ，表示每个子数组的结尾。


• An array of T**, representing the beginning of each sub-array.
• An array of T**, representing the end of each sub-array.

Lo，看到我遇到了 goto 似乎是适当的。

Lo and behold, I ran across a situation where goto seemed to be appropriate.

但是我内心的东西尖叫着不！所以我想我会来这里问：

But something within me screamed "NO!!" so I thought I'd come here and ask:

#include <algorithm>

template<class T>
class lazy_concat_iterator
{
    // This code was meant to work for any valid input iterator
    // but for easier reading, I'll assume the type is: T**

    mutable T** m_endIt;              // points to an array of end-pointers
    mutable T** m_it;                 // points to an array of begin-pointers
    mutable bool m_started;   // have we started iterating?
    mutable T* m_sub;         // points somewhere in the current sub-array
    mutable T* m_subEnd;      // points to the end of the current sub-array

public:
    lazy_concat_iterator(T** begins, T** ends)
        : m_it(begins), m_endIt(ends), m_started(false) { }

    void ensure_started() const
    {
        if (!m_started)
        {
            m_started = true;

        INIT:
            m_sub = *m_it;
            m_subEnd = *m_endIt;

            if (m_sub == m_subEnd)  // End of this subarray?
            {
                ++m_it;
                ++m_endIt;
                goto INIT;  // try next one         <<< should I use goto here?
            }
        }
    }
};

如何使用它：

#include <vector>
#include <cstring>

using namespace std;

int main(int argc, char* argv[])
{
    vector<char*> beginnings(argv, argv + argc);

    vector<char*> endings;
    for (int i = 0; i < argc; i++)
        endings.push_back(argv[i] + strlen(argv[i]));

    lazy_concat_iterator<char> it(&beginnings[0], &endings[0]);
    it.ensure_started();  // 'it' would call this internally, when dereferenced
}

推荐答案

是的，你可以而且应该避免 goto ，例如这段代码应该从 INIT label（这也适用于输入迭代器，这是一个隐藏要求，因为它不引用 m_it 和 m_endIt 一次满足条件的额外时间不像我以前的转换）：

Yes, you can and should avoid goto, for example this code should do the equivalent for what yours does from the INIT label (this also works for input iterators which was a "hidden requirement" as it doesn't dereference m_it and m_endIt an extra time once the condition is met unlike my previous transformation):

while ((m_subIt = *m_it) == (m_subEnd = *m_endIt))
{
    ++m_it;
    ++m_endIt;
}

---

尝试：

即使是永远的循环也会比 goto

Even a forever loop would be clearer and neater than a goto. It highlights the obvious "never terminate" possibility even better.

    for (;;)
    {
        m_sub = *m_it;
        m_subEnd = *m_endIt;

        if (m_sub != m_subEnd)
            break;

        ++m_it;
        ++m_endIt;
    }

虽然我不明白为什么你需要分配 m_subEnd 和 m_subIt 。如果你不能，你可以重写这个while循环：

Although I don't see why you need to assign to m_subEnd and m_subIt inside the loop. If you don't you can rewrite this as a while loop:

while (*m_it == *m_endIt)
{
    ++m_it;
    ++m_endIt;
}

m_subIt = *m_it;
m_subEnd = *m_endIt;

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