You could put it in any format you like, and C++ could parse it. XML is a popular format, but many other programmers despise it (it's love or hate). JSON might be a good bet.
There is no language called 'C+', as far as I know. There is 'C', and there is 'C++'. There is also C#. All three of those could handle whatever format you choose.
However, C and C++ doesn't natively know about the HTTP protocol like PUSH and GET commands. It can certainly handle it, but it requires work on the programmer's side. So if you were intending on starting a business doing this, you should create a C++ library that C++ programmers will use that handles that kind of thing for them - or they'll either have to find a third party library on their own, make one themselves, or just not use your service.
C++ can handle almost anything. But it supports almost nothing on its own, requiring third-party libraries or self-made libraries.