在类 C++ 中使用运算符+= 连接 2 个字符串

问：

在问之前，我见过一些类似的问题，但我仍然停留在使用 .operator+=

目前，我可以通过构造函数方法正确获取单独的字符串。但是当我编译代码时，方法中的行显示错误：str[length+i] = s[i];String& String::operator+= (const String& s)

'operator[]' 不匹配（操作数类型为 'const String' 和 'unsigned int'）

所以我需要你的帮助来修复这个错误。

这是我的代码：

#include <iostream>
#include <cstring>
 
class String {
 
    // Initialise char array
    char* data;
    unsigned length;
 
public:
    // Constructor without arguments 
    String();
 
    // Constructor with 1 arguments
    String(char* s);
 
    // Copy Constructor
    String(const String& source);
 
    // Move Constructor
    String(String&& source);
 
    // Destructor
    ~String() { delete[] data; }
    
    /*! 
     *  @brief String length.
     *  @return Value in String @c length.
     */
    unsigned len ( ) const;
    
    /*! 
     *  @brief Append to String.
     *  @param[in] s A String object.
     *  @return A String reference to *this.
     *  @post String will equal the concatenation of itself with @a s.
     */
    String& operator+= (const String& s);
};
 
// Constructor with no arguments
String::String()
    : data{ nullptr }
{
    data = new char[1];
    data[0] = '\0';
}
 
// Constructor with one arguments
String::String(char* s)
{
    if (s == nullptr) {
        data = new char[1];
        data[0] = '\0';
    }
 
    else {
 
        data = new char[strlen(s) + 1];
 
        // Copy character of s[]
        // using strcpy
        strcpy(data, s);
        data[strlen(s)] = '\0';
 
        std::cout << data << "\n";
    }
}
 
// Copy Constructor
String::String(const String& source)
{
    data = new char[strlen(source.data) + 1];
    strcpy(data, source.data);
    data[strlen(source.data)] = '\0';
}
 
// Move Constructor
String::String(String&& source)
{
    data = source.data;
    source.data = nullptr;
}

unsigned String::len ( ) const 
{
    return length;
}

String& String::operator+= (const String& s) 
{
    unsigned len = length + s.len();
    char*    str = new char[len];

    for (unsigned j=0; j < length; j++)
        str[j] = data[j];

    for (unsigned i=0; i < s.len(); i++)
        str[length+i] = s[i];

    delete data;
    length = len;
    data   = str;
    return *this;    
}
 
int main()
{
    // Constructor with no arguments
    String a;
 
    // Convert string literal to
    // char array
    char temp[] = "Hello world.";
 
    // Constructor with one argument
    std::cout << "s1: ";
    String s1{ temp };
 
    // Copy constructor
    String s11{ a };
 
    char temp1[] = "Goodbye!";
    std::cout << "s2: ";
    String s2{ temp1 };
    
    String s3 = String s1 + String s2;

    return 0;
}

主函数的另一种写法：

int main()
{
String s1("Hello World.");
String s2("Goodbye!");
std::cout << "s1: " << s1 << std::endl;
std::cout << "s2: " << s2 << std::endl;
String s3 = s1 + s2;
std::cout << "s3: " << s3 << std::endl;
std::cout << "The last char of s3: " << s3[s3.size()-1] << std::endl;
return 0;
}

预期结果：

s1: Hello World.
s2: Goodbye!
s3: Hello World.Goodbye!
The last char of s3: !

如何修改我的代码以正确获取和最后一个字符？s3s3