c++写的一个类，重载加号等号无法连用

写了一个简单的String类，重载了加号和等号，等号可以直接一对一赋值，但加了再赋值就不行。代码如下，请指教

 #include<iostream>
#include<cString>
using namespace std;
class String {
    char *str;
    int len;
    public:
        String(char *s = 0) {
            if(s) {
                len = strlen(s);
                str = new char[len + 1];
                strcpy(str, s);
            }
            else {
                len = 0;
                str = 0;
            }
        }
        String(String &s) {
            if(s.str) {
                len = s.len;
                str = new char[len + 1];
                strcpy(str, s.str);
            }
            else {
                len = 0;
                str = 0;
            }
        }
        ~String() {
            if(str != NULL) delete []str;
        }
        void show() {
            if(str) cout<<str<<endl;
            else cout<<"this string is empty"<<endl;
        }
        String &operator =(String s){
            if(str) delete []str;
            if(s.str) {
                len = s.len;
                str = new char[len + 1];
                strcpy(str, s.str);
            }
            else {
                len = 0;
                str = 0;
            }
            return *this;
}
      friend String &operator +(String &, String &);
｝；
String operator +(String &s1, String &s2) {
    String temp;
    temp.len = s1.len + s2.len;
    temp.str = new char[temp.len + 1];
    strcpy(temp.str, s1.str);
    strcat(temp.str, s2.str);
    return temp;
}
int main() {
    char *a="C plus plus ";
    String s1(a), s2("language"), s3;//测试两种构造函数

    s1.show( );
    s2.show( );
    s3 = s1 + s2; //此行报错
    s3.show( );
    ｝

报错为 [Error] no matching function for call to 'String::String(String)'