c++ - 为什么这条线运行两次并且行为异常？

Question

作为序言，我最近才开始自学编程，因此我和他们一样是新手。如果您发现我的代码中有任何不良的一般编码做法(包括与我的问题无关的问题)，请告诉我，因为老实说，我可能不知道更好。

关于问题:

我正在尝试编写一小段代码，用新字符串替换出现的目标字符串。我知道存在可以大大改进过程的函数(即查找、替换等)，出于练习的目的，我避免使用这些函数并将自己限制为使用迭代器、插入和删除。我也知道还有许多其他地方可以改进代码，我也很乐意对这些部分提出意见，但我主要关心的是我发现的异常行为。我现在拥有的是:

#include <iostream>
#include <string>

bool scan(const std::string, const std::string::iterator);

//replaces occurrences of oldVal in s with newVal
std::string& replace (std::string &s, const std::string &oldVal, const std::string &newVal) 
{
    std::string::iterator iter = s.begin();
    while (iter != s.end()) { //process s
        if (scan(oldVal, iter)) { //Checks if characters match
            iter = s.erase(iter, iter + oldVal.size()); //deletes occurrence of oldVal
            iter = s.insert(iter, newVal.begin(), newVal.end()); //inserts newVal in oldVal's place
            iter = iter + newVal.size(); //continues processing after inserted newVal
        }
        else
            ++iter; //Move to next character in s to process
    }
    return s;
}

//This function returns 1 if the strings match and 0 if they do not
bool scan(const std::string target, std::string::iterator iter)
{
    for (auto beg = target.begin(); beg != target.end(); ++beg) {
        std::cout << "Line 27 " << *iter << " " << *beg << std::endl; //MAIN CONCERN! This line is run twice. 
                                                                      //(It was solely added for debugging. Verifies values being compared)
        if (*iter != *beg) {
            std::cout << "Condition met; " 
                      << *iter << " != " << *beg << std::endl; //added for debugging. Double verifies values post-comparison
            return 0;
            }
        ++iter;
        ++beg;
    }
    return 1;
}

int main()
{
    std::string mainStr, oldStr, newStr;
    std::getline(std::cin, mainStr); //Overall it'd be better for s to be a list of strings, but my concern is with line 27
    std::cin.clear();
    std::cin >> oldStr >> newStr;
    std::cout << "Output: " << replace(mainStr, oldStr, newStr) << std::endl; //Prints post-replacement string
}

似乎正在发生的是第 27 行 (std::cout << "Line 27 "...) 在第一次调用扫描时执行了两次。例如，给定输入:

tho
tho though

我得到了输出(//是我在运行之外添加的评论，即仅针对这篇文章)

Line 27 t t //This part is run twice and does weird things
Line 27 h o //If it was just incrementing it should read "Line 27 h h"
Condition met; h != o //This shouldn't happen; The condition should have tested t != t
Line 27 h t //It's seems fine from this point onwards
Condition failed; h != t
Line 27 o t
Condition failed; o != t
Output: tho

这可能是什么原因？

谢谢!

Answer 1

我发现的一个错误:在 scan() 中，beg 迭代器在每次迭代中增加两次，因为 ++beg 在for(...) 循环行和 beg++ 在循环的末尾。