制作一个执行基本字符串操作的字符串类。代码无法正常运行

Question

我正在尝试创建自己的字符串类，该类执行一些基本的字符串操作。但是，我的代码无法正常工作。它可以运行，但不输出任何内容，而且我似乎找不到错误。任何帮助将不胜感激。下面的代码：

.h文件-

    #ifndef PART_2_H_INCLUDED
    #define PART_2_H_INCLUDED

    class String
    {
    public:
        int str_size;
        char *str;

        String() { str = nullptr; str_size = 0; }
        String(const char *usrstr);
        String(String &usrstr);

        String& operator = (const String& usrstr);
        String operator + (const String& usrstr1);

        char *c_str();

        int length();

        void clear();

        ~String() { delete [] str; }
    };

    #endif // PART_2_H_INCLUDED

.cpp文件-

#include <iostream>
#include "Part_2.h"
#include <cstring>

using namespace std;

int String::length()
{
    int i = 0;
    while(str[i] != '\0')
        i++;

    return i;
}

String::String(const char *usrstr)
{

    int i = 0;
    while(usrstr[i] != str_size)
    {
      i++;
    }

    int usrstrlen = i;
    this->str_size = usrstrlen;
    str = new char[str_size];

    i = 0;
    while(i < this->str_size)
    {
      this->str[i] = usrstr[i];
      i++;
    }
}

String::String(String &usrstr)
{
    int i = 0;
    this->str_size = usrstr.str_size;
    str = new char[this->str_size];

    while(i < this->str_size)
    {
        this->str[i] = usrstr.str[i];
        i++;
    }
}

String& String::operator = (const String& usrstr)
{
    int i = 0;
    this->str_size = usrstr.str_size;

    this->str = new char[this->str_size];

    while(i < this->str_size)
    {
        this->str[i] = usrstr.str[i];
        i++;
    }

  return *this;
}

String String::operator + (const String& usrstr1)
{
    String obj;
    int i = 0;
    int j = 0;

    obj.str_size = this->str_size + usrstr1.str_size;

    obj.str = new char[obj.str_size];

    while(i < this->str_size)
    {
        obj.str[i] = this->str[i];
        i++;
    }

    while(j < usrstr1.str_size)
    {
        obj.str[i] = usrstr1.str[j];
        i++;
        j++;
    }

    cout << obj.str;

    return obj;
}

char* String::c_str()
{
    char *cstring = new char[this->str_size + 1];

    int i = 0;
    while(i < this->str_size)
    {
        cstring[i] = this->str[i];
        i++;
    }

    cstring[i] = '\0';

    return cstring;
}

void String::clear()
{
    int i = 0;
    while(i < this->str_size)
    {
        this->str[i] = '\0';
        i++;
    }

    this->str_size = 0;
}


int main(){

String obj1("Testing");

String obj2(obj1);

String obj3=obj2;

cout << obj1.str << endl;

cout << obj3.str << endl;
obj3.clear();
cout << obj3.str;

cout << obj1.length();

return 0;
}

对于任何菜鸟错误我深表歉意。