c++字符串问题

Question

代码如下图所示，在main函数中，有两个string类型的变量，其中一个变量str1我直接初始化，另一个变量str2我没有初始化，但是string类型调用默认构造函数，使得str2 = "\0",是这样吧。我对str1的首字符进行了变换，然后用cout可以在后面输出，为什么我对str2的首字符进行变换后，再用cout输出，但是没有得到正确的输出啊。请问这是怎么回事啊？谢谢。

#include <iostream>
#include <string>

int main()
{
    std::string str1 = "test";
    std::string str2;

    if (str2[0] == '\0')
        std::cout << "yes" << std::endl;

    str1[0] = 'a';
    str2[0] = 'b';
    std::cout << str1 << '\n'
              << str2 << '\n';
    
    str2[1] ='\0';

    std::cout << str1 << '\n'
              << str2 << '\n';

    return 0;
}

运行结果：

Answer 1

#include <iostream>
#include <string>

int main()
{
    std::string str1 = "test";    //有效长度4（不包含'rrreee'）
    std::string str2;    //有效长度0

    //前面声明其实和 str1[4], str2[0]没什么不同 
    
    // c++ string类并没有智能到 像其他的一些语言
    // 你访问没有分配的字符下标 自动分配内存的地步 
    // 正如str2[0] 你并没有分配任何内存，也就不能访问了

    if (str2[0] == 'rrreee')   //这个访问是 错误的，如果你判断是否为空请用 .empty
        std::cout << "yes" << std::endl;

    str1[0] = 'a';    //如果你非要这么做可以使用 .insert的重载
    str2[0] = 'b';
    std::cout << str1 << '\n'
              << str2 << '\n';
    
    str2[1] ='rrreee';

    std::cout << str1 << '\n'
              << str2 << '\n';

    return 0;
}

Answer 2

According to your question.
You created str1, which points to five bytes.
And str2 is empty.
So
you use str1[0]str1[1 ]str1[2]str1[3]str1[4] is fine
But there will be a problem if you use str2[0]. It actually does not point to any memory.
Personal suggestion.
You use C++, it is not recommended to use array subscripts in C language to access.
It would be better to use some functions that come with string to access.

Answer 3

When you output, try outputting str2.data() instead.
str2[0] = 'b'; Should this operation be effective? This question is worth thinking about
because currently str2 is an empty string, that is, empty()==true;
then its size() should be 0, and str2[0] is for the first element access.

This is actually related to the implementation of std::string. You can try it. Some compilers’ built-in implementations will cause errors during this operation.

/usr/local/lib/gcc/i686-pc-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/basic_string.h:707: typename _Alloc::rebind<_CharT>::other::reference std::basic_string<_CharT, _Traits, _Alloc>::operator[](typename _Alloc::rebind<_CharT>::other::size_type) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]: Assertion '__pos < size()' failed.

Disallowed system call: SYS_kill