我的问题与下面的线程相同，我很难理解给出的答案，或者更确切地说，我的代码不应该工作，因为它只使用输入迭代器..但我的 func 似乎工作并且行为与 std 相同: :search ..所以我很茫然，不愿意在没有正确理解的情况下继续前进......也许如果有人可以提出一个会破坏我的功能但不会破坏 std::

来自 Why do I need a Forward Iterator to implement my customized std::search :

I am studying the book "Accelerated C++" from Koenig & Moo.

Exercise 8-2 ask me to implement on my own some templatized functions from <algorithm> and <numeric>, and to specify what kind of iterator does my implementation require.

When trying to implement std::search, I determined that I need only "input" iterators.

However, looking at the implementation of std::search installed with my compiler, I can see that they use "forward" iterators, but I cannot understand why, because there is no need to write, only to read, and input iterators meet the requirement.

Can anybody here help me to understand this, please? Why would I need to use "forward" iterators to implement std::search?

Thanks in advance.

我的函数:

template <class In> 
In search(  In begin, In end, In begin2, In end2 )
{
    In found ;                      // iter: 1st element in pattern match(in content)
    In pattern_begin = begin2 ;     // iter: 1st element in search pattern.
    int flag = 0 ;                  // flag: partial match found?

    // search content for pattern 
    while (  begin < end  ) {

        // if pattern-match fails ..reset vars
        // & continue searching remaining content/elements
        if ( *begin != *begin2 ) {

            In ret ;                     
            begin2 = pattern_begin ;
            flag = 0 ;
            begin++ ;


        } else {
            // compare next element in pattern with next element in content.
            // if: 1st element of 'pattern' is found, store iter to it's pos
            // ..then if entire pattern is found, we can ret an iter to where it starts
            if ( flag == 0 ) { 
                found = begin ;
                flag = 1 ;
            }
            // inc iters to compare next elements in partial match
            begin++ ;
            begin2++ ;
        }

        // if: iter is 1-past end of search pattern
        // then entire pattern has been found 
        // return the iter to where it starts
        if( begin2 == end2 ) {  return found ;  }

    }

    // end of content reached, no complete pattern found
    // begin should? equal an iter 1-past the end of content
    return begin ;
}

司机:

///* // Driver: custom::search(  b, e, b2, e2  ) 
#include <string>
#include <vector>
#include <iostream>
//#include <algorithm>
#include "library_algorithms.h"

int main() {

    // init string test
    std::string content = "fo The fox  foxu jumped  foxe foxy " ;
    std::string search_pattern = "foxy" ;

    // func test on string
    std::string::iterator ret_iter = 
    custom::search(  content.begin(), content.end(), search_pattern.begin(), search_pattern.end()  ) ;
    //std::search(  content.begin(), content.end(), search_pattern.begin(), search_pattern.end()  ) ;

    // output
    if (  ret_iter != content.end()  ) {

        std::cout << std::endl << std::endl << search_pattern << ": found at position: " << int(  ret_iter - content.begin()  ) << std::endl;

    } else { 

        std::cout << std::endl << std::endl << search_pattern << ": ...not found" << std::endl;
    }




    // Init vec test:
    // create content values in range:  10 20 30 <......> 9970 9980 9990
    std::vector<int> myvector;
    for (int i=1; i<1000; i++) myvector.push_back(i*10);

    // create pattern values to search for
    std::vector<int> pattern ;
    pattern.push_back( 3730 ) ;
    pattern.push_back( 3740 ) ;
    pattern.push_back( 3750 ) ;
    pattern.push_back( 3760 ) ;

    // test: func on vector<int>
    std::vector<int>::iterator it;
    it = custom::search (  myvector.begin(), myvector.end(), pattern.begin(), pattern.end() );

    // output
    if (it!=myvector.end())
    std::cout << std::endl << std::endl << "pattern found at position " << int(it-myvector.begin()) << std::endl;
    else
    std::cout << std::endl << std::endl << "pattern not found" << std::endl;





    return 0 ;

}

最佳答案

您误解了输入迭代器的功能。

您不能“保存”或复制输入迭代器。它允许您恰好遍历序列一次。换句话说，这一行以及其他行将中断:begin2 = pattern_begin。

输入迭代器可能表示您无法轻易“倒带”的内容，例如，从网络适配器接收的数据流。指向“6 个元素之前”的迭代器不再有意义，因为内存中可能不再提供该数据。您在信息流中只有当前位置。

很明显，为了正确地实现search，您需要能够多次遍历序列的各个部分。

关于c++ - 为什么 std::search 需要转发迭代器，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/5368460/