c++ - 较长的 getline cin 输入问题

Question

我正在编写一个 C++ 程序，它要求用户输入一个单词或句子，检查单词/句子，用“aoa”或“AoA”替换“a”或“A”的所有实例，然后输出结果。但是，如果我尝试输入更长的句子，就会遇到问题。例如，如果我输入“为什么程序不正常”，程序会输出奇怪的字母而不是预期的结果。这是我的代码:

#include <iostream>
#include <string>

using namespace std;

int main(int argc, const char * argv[])
{
string mening, temp; //The mening string is the word/sentence the user will input.
int play = 1, add;

while (play == 1) {
cout<<"Type in the sentence: ";

getline(cin, mening); //The input is saved in the string variable mening.

unsigned long y = mening.size(); //Grabs the amount of characters in input; this number is saved in the unsigned long variable y.
add = 0; //Makes sure the int variable add is reset to 0 if the loop restarts.

for (int k = 0, n = 1;n<=y;k++, n++) {
    if (mening[k] == 'a' || mening[k] == 'A') {
        k++;


        for (int i = k, m = 1;m<=y - n;i++, m++) {
            temp[i] = mening[i];
        } //The characters after the one that has been checked are stored in temp array indexes, if the character that has been checked is an a or A.

        for (int i = k, m = 1, j = k + 2;m<=y - n;i++, m++, j++) {
            mening[j] = temp[i];
        } //The characters after the one that has been checked move two steps to the right, to allow the two extra letters.

        mening[k] = 'o';
        mening[k + 1] = mening[k - 1];

        k++;

        add = add + 2; //The int variable add is increased by 2 during each aoa/AoA to avoid strange characters being outputted at the very end.

    }
    else { }

}

for (int k = 0;k<=y + add - 1;k++) {
    cout<<mening[k];
}

cout<<endl<<"Do you want to do it again? (yes/no): ";

getline(cin, mening);

    cin.clear();
    cout << flush;
    cout.flush();
    cout.clear();

if (mening == "Yes" || mening == "yes" || mening == "YES") {

}
else {
    play = 2;
}
}


cout<<endl<<"The program will now close.";

return 0;
}

可能导致问题的原因是什么？

Answer 1

一个紧迫的问题是您从 [1,n] 建立索引，而 C++(std::string、std::vector，还有 C 风格数组)使用 [0,n)。这意味着您将可以访问字符串的结尾。你正在将字符存储到 temp使用 temp[x]，尽管 temp 的大小始终为 0。两者这些是未定义的行为，可能会产生任何影响(包括使程序崩溃)。

你应该使用标准库的 Debug模式开发代码。在 Visual Studios 中，我认为这是默认;使用 g++，您需要添加 -D_GLIBCXX_CONCEPT_CHECKS
-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC 到你的命令线。

处理这个问题的最简单方法是复制到一个新字符串中，随时进行更改:

std::string results;
for ( auto current = mening.cbegin(); current != mening.cend(); ++ current ) {
    switch ( *current ) {
    case 'a':
        results += "aoa";
        break;

    case 'A':
        results += "AoA";
        break;

    default:
        results += *current;
        break;
    }
}

如果您确实想就地进行替换，就会变得很棘手。当插入的文本多于开始时，迭代器是无效。所以你需要这样的东西:

static std::string const Ao( "Ao" );
static std::string const ao( "ao" );
for ( auto current = mening.begin(); current != mening.end(); ++ current ) {
    switch ( *current ) {
    case 'a':
        current = mening.insert( current, ao.begin(), ao.end() ) + 2;
        break;

    case 'A':
        current = mening.insert( current, Ao.begin(), Ao.end() ) + 2;
        break;
    }
}

就我个人而言，我更喜欢复制到一个新字符串中。