定义字符串连接的友元函数有问题，怎么改啊
#include <iostream>
#include <string.h>
#include <assert.h>

using namespace std;

class String
{
private:
    int length;
    char* str;
public:
    String();
    String(char* str0);
    String(const String& str0);
    int size();
    friend istream& operator>>(istream& in,String& str0);
    friend ostream& operator<<(ostream& out,String& str0);
    char& operator[](int index);
    以下四个函数的实现有问题，得不到想要的结果，输出乱码
    friend String& operator+(const String& str0,const String& str1);
    friend String& operator+(const String& str0,const char* str1);
    friend String& operator+=(String& str0,const String& str1);
    friend String& operator+=(String& str0,const char* str1);
    ~String();
};

String::String()
{
    length=1;
    str=new char[length+1];
    str[0]='A';
}

String::String(char* str0)
{
    int len=0;
    for(int i=0;str0[i]!='\0';i++)
    {
        len++;
    }
    length=len;
    str=new char[length+1];
    assert(str!='\0');
    strcpy(str,str0);
}

String::String(const String& str0)
{
    length=str0.length;
    str=new char[length+1];
    str=str0.str;
}

int String::size()
{
    return this->length;
}

istream& operator>>(istream& in,String& str0)
{
    in>>str0.str;
    str0.length=strlen(str0.str);
    return in;
}

ostream& operator<<(ostream& out,String& str0)
{
    out<<str0.str;
    return out;
}

char& String::operator[](int index)
{
    if(index>=0&&index<=size())
    {
        return this->str[index];
    }
}

String& operator+(const String& str0,const String& str1)
{
    String str2;
    str2.length=str1.length+str0.length;
    str2.str=new char[str2.length+1];
    strcpy(str2.str,str0.str);
    strcat(str2.str,str1.str);
    return str2;
}

String& operator+(const String& str0,const char* str1)
{
    String str2;
    str2.length=strlen(str1)+str0.length;
    str2.str=new char[str2.length+1];
    strcpy(str2.str,str0.str);
    strcat(str2.str,str1);
    return str2;
}

String& operator+=(String& str0,const String& str1)
{
    String str2;
    str2.length=str1.length+str0.length;
    str2.str=new char[str2.length+1];
    strcpy(str2.str,str0.str);
    strcat(str2.str,str1.str);
    str0=str2;
    return str0;
}

String& operator+=(String& str0,const char* str1)
{
    String str2;
    str2.length=strlen(str1)+str0.length;
    str2.str=new char[str2.length+1];
    strcpy(str2.str,str0.str);
    strcat(str2.str,str1);
    str0=str2;
    return str0;
}

String::~String()
{
    delete[] this->str;
}

int main()
{
    String k;
    cin>>k;
    cout<<k<<endl;
    cout<<k.size()<<endl;

    String x("ZhangQian");
    cout<<x<<endl;
    cout<<x.size()<<endl;

    String y(x);
    cout<<y<<endl;
    cout<<y.size()<<endl;

    String a="WangYun";
    cout<<a<<endl;
    cout<<a.size()<<endl;

    String b=x;
    cout<<b<<endl;
    cout<<b.size()<<endl;

    char c=x[1];
    cout<<c<<endl;
    cout<<x<<endl;

    a[2]='c';
    cout<<y<<endl;
    cout<<a<<endl;

    String aa=x+k;
    cout<<aa<<endl;
    cout<<aa.size()<<endl;

    char* ddd="SuXiaoLing";
    String dddd=k+ddd;
    cout<<dddd<<endl;
    cout<<dddd.size()<<endl;

    String zhangqian="ZhangQian";
    zhangqian+=dddd;
    cout<<zhangqian<<endl;

    zhangqian+="wangyun";
    cout<<zhangqian<<endl;
    cout<<zhangqian.size()<<endl;

    return 0;
}

输出结果

谢谢大神赐教