is C++ fully standardised & portable ?
I know there is boost (http://www.boost.org) which will be the future
standard of C++. But what is the possibility that std C++ be portable
as C programming language ? Is it mature enough at the moment to say
C++ is completely portable and it can be coded in such a way as C
programs which can be ported across to almost every platform with or
without less efforts or changes to the source code ? This is more or
less a generalized query.

The issue with C++ is that for many years most C++ compilers were not
compliant to the C++ standard, as parts of it were difficult to
implement, or for other reasons. It is now possible to get full
implementations of C++ compilers for many platforms, but if you start
using things like namespaces, some platforms without fully-compliant
compilers are not going to be able to compile it.

The primary issue with portability in C programs is that you have to
program intelligently (if you stay out of C99 territory.) In my
experience, the problem with C++ was that the compiler didn't
implement the full spec.

Boost is providing an answer to a different problem, which is
providing portable implementations of common programming abstractions.
If you don't have a fully compliant compiler, Boost still won't help
you. Please note the following message on their web site, which says
it better than I can: "If you don't have a compliant compiler, I'm
sure some stuff in Boost still isn't going to work."

I didn't answer all your question, but I hope you find the information useful.

That was the wrong quote. Here is the correct one:

"Since Boost libraries rely on modern C++ features not available in
all compilers, not all Boost libraries will work with every compiler."