[INTERFACES] <string> in pgconnection.h (?)

I'm putting together my first C++ pgsql client program and have run into
a snag. I #include <pgdatabase.h>, but when I compile, I get a message
saying

/usr/include/pgsql/libpq++/pgconnection.h:24: string: No such file or
directory

Looking in pgconnection.h, I see

#include <string>

(Call me old-fashioned, but I thought us C hacks had agreed to use ".h"
when naming our include files.) Later on in the file I see the
declaration

string IntToString(int);

Now, I assume that "string" is a class. But where would I find the
header file "string"?

Thanks in advance for clarifying what is clearly !obvious to me.

Cheers,
Richard Blackwell