duplicate and deescape string

destrndup

char* destrndup(const char*s, size_t n){
  if (s==NULL)return NULL;
  char *ret(new char[n+1]),*t(ret),*p;
  const char *e(&s[n]);
  for(;*s!='\0'&&s!=e&&n>0;--n,++s,++t){
//    printf("estrndup: s=%c\n",*s);
    if(*s=='\\'){
      ++s;--n;
      switch(*s){
        case 'a':*t='\a';break;
        case 'b':*t='\b';break;
        case 'f':*t='\f';break;
        case 'n':*t='\n';break;
        case 'r':*t='\r';break;
        case 't':*t='\t';break;
        case 'v':*t='\v';break;
        case '?':*t='\?';break;
        case '\\':*t='\\';break;
        case '\'':*t='\'';break;
        case '\"':*t='\"';break;
        case 'x':*t=(char)strtol(s+1,&p,16);s=p-1;break;
        default:
          if(*s>='0'&&*s<='7'){ *t=strtol(s,&p,8);s=p-1;break; }
          else {
            delete ret; 
            throw std::logic_error("estrndup: contained wrong escaped character");
          }
      }
    }else if(*s=='"'){
      delete ret;
      throw std::logic_error("estrndup: contained un-escaped '\"'");
    }else{
      *t=*s;
    }
  }
  *t='\0';
  return ret;
}