C++关于运算符重载的问题

刚做了一道C++编程题，题目是定义一个字符串类STR，实现字符串的拼接，我的代码如下，但是输出没有达到目的，哪位大神可以解释一下原因？

#include <iostream>
using namespace std;
class STR
{
private:
    char s[50];
public:
    STR(char p[50]) { strcpy(s, p); }
    void print() { cout << s << endl; }
    friend STR& operator+=(STR str1, STR str2);
};
STR& operator+=(STR str1, STR str2)
{
    return(STR(strcat(str1.s, str2.s)));
}
int main() 
{
    STR s1("Hello "), s2("World!");
    s1 += s2;
    s1.print();
    system("pause");
    return 0;
}

#include <iostream>
using namespace std;
class STR
{
private:
    char s[50];
public:
    STR(char p[50]) { strcpy(s, p); }
    void print() { cout << s << endl; }
    friend STR& operator+=(STR& str1, STR str2);
};
STR& operator+=(STR& str1, STR str2)
{
    return(STR(strcat(str1.s, str2.s)));
}
int main() 
{
    STR s1("Hello "), s2("World!");
    s1 += s2;
    s1.print();
    system("pause");
    return 0;
}

或者

#include <iostream>
using namespace std;
class STR
{
private:
    char* s;
public:
    STR(char * p) { s = new char[50]; strcpy(s, p); }
    STR(STR& s2) { s = s2.s; }
    void print() { cout << s << endl; }
    friend STR& operator+=(STR str1, STR str2);
};
STR& operator+=(STR str1, STR str2)
{
    return(STR(strcat(str1.s, str2.s)));
}
int main() 
{
    STR s1("Hello "), s2("World!");
    s1 += s2;
    s1.print();
    system("pause");
    return 0;
}

又或者

#include <iostream>
using namespace std;
class STR
{
private:
    char s[50];
public:
    STR(char p[50]) { strcpy(s, p); }
    void print() { cout << s << endl; }
    friend STR& operator+=(STR str1, STR str2);
};
STR& operator+=(STR str1, STR str2)
{
    return(STR(strcat(str1.s, str2.s)));
}
int main() 
{
    STR s1("Hello "), s2("World!");
    s1 = s1 += s2;
    s1.print();
    system("pause");
    return 0;
}

#include <iostream>
using namespace std;
class STR
{
private:
    char s[50];
public:
    STR(char p[50]) { strcpy(s, p); }
    void print() { cout << s << endl; }
    STR& operator+=(STR str2)
    {
        return(STR(strcat(s, str2.s)));
    }
};

int main() 
{
    STR s1("Hello "), s2("World!");
    s1 += s2;
    s1.print();
    system("pause");
    return 0;
}

办法很多很多的