字符串反转赋值遇string subscript out of range错误，求解决方案

Hey there! Let's break down why you're hitting that string subscript out of range error and how to fix it.

The Root Cause #

When you initialize string b;, it starts as an empty string with a size of 0. In your loop, you're trying to assign values to b[i] directly—but since b has no allocated space for those characters yet, accessing b[i] is out of bounds. Unlike some other languages, C++'s std::string doesn't automatically expand when you use the [] operator; you have to make sure the string has enough space first.

Solution 1: Pre-allocate Space for b #

Before your loop, use resize() to give b the same length as a. This ensures there's a valid spot for every character you want to assign:

#include <iostream>
using namespace std;
int main() {
    string a, b;
    cin >> a;
    // Resize b to match a's length so we can safely use []
    b.resize(a.size());
    for (int i = 0; i < a.size(); i++) {
        b[i] = a[a.size() - 1 - i];
    }
    cout << b;
}

Solution 2: Use push_back() to Build b #

Instead of pre-allocating space, you can use push_back() to add characters to the end of b one by one. This method automatically expands the string's size as needed, so you don't have to worry about bounds at all:

#include <iostream>
using namespace std;
int main() {
    string a, b;
    cin >> a;
    for (int i = 0; i < a.size(); i++) {
        // Add each reversed character to the end of b
        b.push_back(a[a.size() - 1 - i]);
    }
    cout << b;
}

A Quick Note on "No Built-in Functions" #

Since you mentioned wanting to avoid built-in reversal tools (like std::reverse), both solutions above stick to basic string operations without using specialized reversal functions. The resize() and push_back() methods are core std::string members that just handle space management—they don't do any of the reversal work for you, which fits your goal perfectly.

内容的提问来源于stack exchange，提问作者PrinceEGY