C++分配后字符串大小未更新

operator[] 不会调整字符串的大小。你用索引 5、6 和 7 对它的调用超出了范围和未定义的行为。使用 resize 将字符串设置为特定大小，或者push_back或operator+=追加字符。

另请注意，您不需要手动将std::string端零。类将自行处理。尽管如果您真的想要它们，则可以在其中嵌入零，并且它们将被视为字符串长度的一部分。

s2.reserve(10);根本不会增加字符串，它只是告诉容器为至少 10 个字符保留足够的内存。它不会用任何东西填充保留的空间。

因此，当你索引它时s2[5]你基本上索引在"使用"字符串的边界之外（即它的大小），它是未定义的行为。

要调整大小，可以使用 s2.resize(10); 。这将适当地分配和填充字符串，它的大小为 10 。要同时分配和插入字符，您还可以使用 push_back() 或 operator+= .

附带说明：不需要s2[7] = '';。string类在内部管理方法（如c_str()等）所需的任何 NUL 终止。您无需自己添加 NULL。

你应该使用s2.resize()而不是s2.reserve()。

std::string::reserve只分配内存，但不调整字符串的大小。在您的示例中：

s2 = s1;         // Resize string to 6 characters
s2.reserve(10);  // Allocate another 4 char, but not resize
s2[5] = '.';     // Write '.' to some memory, but the string is still not resized.

简单的解决方法是使用std::string::resize而不是reserve。

简短回答：使用调整大小（10）而不是保留（10）

长答案：在 std：：string 的实现中，有两个变量大小和容量。容量是已为字符串分配的内存量。大小是字符串中允许的有效元素（在您的情况下为 char）的数量。请注意，容量将始终小于或等于大小。当您调用 reserve（） 时，您正在更改容量。当您调用 resize（） 时，您可能不仅会更改大小，而且如果大小>容量，您还将更改容量，然后应用此公式：

if (size > capacity){
capacity = max(size, capacity*2); //Why multiply capacity by 2 here? This is to achieve amortized O(1) while resizing
}

下面是 OP 想要的代码示例，以及更多代码，以便更好地解释大小和容量

#include <iostream>
#include <string.h>
using namespace std;
int main(int argc, char const *argv[])
{   const char *s1 = "hello";
    string s2;
    s2 = s1;
    cout << "length of s2 before reserve: " << s2.length() << endl;
    cout << "capacity of s2 before reserve: " << s2.capacity() << endl;
    s2.reserve(10);
    cout << "length of s2 after reserve: " << s2.length() << endl; //see how length of s2 didn't change?
    cout << "capacity of s2 after reserve: " << s2.capacity() << endl;
    s2.resize(8); //resize(10) works too, but it seems like OP you only need enough size for 8 elements
    cout << "length of s2 after resize: " << s2.length() << endl; //size changed
    cout << "capacity of s2 after resize: " << s2.capacity() << endl; //capacity didn't change because size <= capacity
    s2[5] = '.';
    s2[6] = 'o';
    s2[7] = '';
    cout << "[" << s1 << "] [" << s2 << "]" << endl;
    // You're done
    // The code below is for showing you how size and capacity works.
    s2.append("hii"); // calls s2.resize(11), s[8] = 'h', s[9] = 'i', s[10] = 'i', size = 8 + 3 = 11
    cout << "length of s2 after appending: " << s2.length() << endl; // size = 11 
    cout << "capacity of s2 after appending: " << s2.capacity() << endl; //since size > capacity, but <= 2*capacity, capacity = 2*capacity 
    cout << "After appending: [" << s1 << "] [" << s2 << "]" << endl;
    return 0;

结果：

length of s2 before reserve: 5
capacity of s2 before reserve: 5
length of s2 after reserve: 5
capacity of s2 after reserve: 10
length of s2 after resize: 8
capacity of s2 after resize: 10
[hello] [hello.o]
length of s2 after appending: 11
capacity of s2 after appending: 20
After appending: [hello] [hello.ohii]