c++ - 在C++中，我遇到无法理解的编译器错误

Question

这是我的代码，它只是颠倒了句子:

#include <iostream>
#include <string>   

using namespace std;

int main()
{
    string sentence;
    string reversedSentence;
    int i2 = 0;

    cout << "Type in a sentence..." << endl;
    getline(cin, sentence);

    for (int i = sentence.length() - 1; i < sentence.length(); i--)
    {
        reversedSentence[i2] = sentence[i];
        i2++;
    }

    cout << reversedSentence << endl;
}

编译工作正常，但是当我尝试运行程序时，会发生这种情况:

Type in a sentence...
[input]
/home/keith/builds/mingw/gcc-9.2.0-mingw32-cross-native/mingw32/libstdc++-v3/include/bits/basic_string.h:1067: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference = char&; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = unsigned int]: Assertion '__pos <= size()' failed.

Answer 1

您的reversedSentence字符串为空，因此对其进行索引会调用未定义的行为。相反，您可以像这样使用push_back:

for (int i = sentence.length() - 1; i >= 0; i--)
{
    reversedSentence.push_back(sentence[i]);
}

另请注意，您的循环条件需要修改。如果sentence为空，则应先将static_cast .length()编码为int，然后再减去1，如下所示:

for (int i = static_cast<int>(sentence.length()) - 1; i >= 0; i--)
{
    reversedSentence.push_back(sentence[i]);
}

您也可以为此使用算法:

reversedSentence = sentence;
std::reverse(reversedSentence.begin(), reversedSentence.end());

这样可以避免在sentence字符串为空时带来麻烦。