string class for C

Design a string ``class'' for C by using structs and function pointers

typedef struct{
char *str;
int len;
}string;

string operator=(string,char *str){
string t;
t.str=str;
t.len=strlen(str);
}

char * operator=(char *,string str){
char *t=malloc(str.len+1);
memcpy(t,str.str,str.len);
t[str.len]=0;
return t;
}

string operator+(string a1,string a2){
string t;
char *tmp=malloc(a1.len+a2.len+2);
memset(tmp,0,a1.len+a2.len+2);
memcpy(tmp,a1.str,a1.len);
memcpy(tmp+strlen(tmp)+1,a2.str,a2.len);
t.len=strlen(tmp);
t.str=tmp;
return tmp;
}