I spent a little while playing around with this.

I tried replacing this line with the following:
```
ss_.operator<<(obj);
```
but ss_.operator<< couldn't find a known conversion for parameter 'const std::basic_string<char>'

So then I tried replacing the line with:
```
std::operator<<(ss_, obj);
```
and it threw an error saying the call was ambiguous.

I also tried adding iostream and string as includes but that didn't affect anything. I tried adding "using" lines but couldn't get it to work although more experimentation here might be worth it.