There are a lot of options, including C-style sprintf(), C++ ostringstream objects, etc. FWIW, itoa() is not a standard function, but a lot of systems and compilers support it - caveat programmer! Anyway, here is a link to some documentation on the ostringstream class which is probably what you should use: http://www.cplusplus.com/reference/iostream/ostringstream/
Not a problem. We all get used to the goodies on our particular systems, often not considering if they are available elsewhere. After 30 years of serious cross-platform development (one code base, 10+ operating systems, 10M lines of code) I quickly learned that what works on one system, may either not work at all (not available), or function differently on another. It gets really fun when you are bit-twiddling and you switch from a big-endian to a little-endian (or vice versa) machine, or one where a long integer is 32 bits to one where it is 64 bits.