在C ++中有一个标准的循环迭代器 [英] Is there a standard Cyclic Iterator in C++

问题描述

基于以下问题：检查一个字符串是否是其他字符串的旋转

我在想一个循环迭代器类型，它接受一个范围，并且能够解决上面的问题是这样的：

I was thinking of making a cyclic iterator type that takes a range, and would be able to solve the above problem like so:

std::string s1 = "abc" ;
std::string s2 = "bca" ;
std::size_t n = 2; // number of cycles
cyclic_iterator it(s2.begin(),s2.end(),n);
cyclic_iterator end;

if (std::search(it, end, s1.begin(),s1.end()) != end)
{
   std::cout << "s1 is a rotation of s2" << std::endl;
}

我的问题，我已经检查过Boost和STL，也没有确切的实现。

My question, Is there already something like this available? I've checked Boost and STL and neither have an exact implementation.

我有一个简单的手工写的（从std :: forward_iterator_tag的专门版本的std :: iterator派生），但宁愿使用已经/实现。

I've got a simple hand-written (derived from a std::forward_iterator_tag specialised version of std::iterator) one but would rather use an already made/tested implementation.

推荐答案

标准中没有这样的。循环不能很好地与C ++迭代器，因为代表整个循环的序列会有 first == last ，因此是空序列。

There is nothing like this in the standard. Cycles don't play well with C++ iterators because a sequence representing the entire cycle would have first == last and hence be the empty sequence.

可能你可以在迭代器中引入一些状态，一个布尔标志来表示尚未完成。旗子参与比较。在迭代之前设置 true ，在递增/递减时设置为 false 。

Possibly you could introduce some state into the iterator, a Boolean flag to represent "not done yet." The flag participates in comparison. Set it true before iterating and to false upon increment/decrement.

但是它可能只是更好地手动编写所需的算法。

But it might just be better to manually write the algorithms you need. Once you've managed to represent the whole cycle, representing an empty sequence might have become impossible.

编辑：现在我注意到您指定了循环次数。这有很大的区别。

Now I notice that you specified the number of cycles. That makes a big difference.

template< class I >
class cyclic_iterator
 /* : public iterator< bidirectional, yadda yadda > */ {
    I it, beg, end;
    int cnt;
    cyclic_iterator( int c, I f, I l )
        : it( f ), beg( f ), end( l ), cnt( c ) {}
public:
    cyclic_iterator() : it(), beg(), end(), cnt() {}

    cyclic_iterator &operator++() {
        ++ it;
        if ( it == end ) {
            ++ cnt;
            it = beg;
        }
    } // etc for --, post-operations

    friend bool operator==
        ( cyclic_iterator const &lhs, cyclic_iterator const &rhs )
        { return lhs.it == rhs.it && lhs.cnt == rhs.cnt; } // etc for !=

    friend pair< cyclic_iterator, cyclic_iterator > cycle_range
        ( int c, I f, I l ) {//factory function, better style outside this scope
        return make_pair( cyclic_iterator( 0, f, l ),
                          cyclic_iterator( c, f, l ) );
    }
};

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