[SOLVED] In C++, I'm getting a compiler error that I can't make sense of

Issue

Here's my code, it just reverses the sentence:

#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;
}

Compilation works fine, but when I try to run the program, this happens:

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.

Solution

Your reversedSentence string is empty, so indexing into it invokes undefined behavior. Instead, you can use push_back like this:

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

Also note that your loop condition needs to be modified. In case sentence is empty, you should static_cast the .length() to an int before subtracting by 1, like this:

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

You could also just use an algorithm for this:

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

This avoids complications when the sentence string is empty.

Answered By - cigien