FreeBASIC is a 32/64-bit, cross-platform, Microsoft QuickBasic compatible, modern object-orientated language in the BASIC family and has headers for numerous add-on libraries. It has official ports for Windows, Linux, and other modern Unices including FreeBSD, OpenBSD, etc, and 32-bit DOS. We use a fork of FB with more complete support for Mac OS and Android.
Think of FB as a subset of C++ with BASIC syntax and some additions & improvements -- FB has equivalents of nearly all features of C++98 except templates. It even has a significantly better-designed reimplementation of the C preprocessor, and easy to use strings.
FreeBASIC has four different dialects, including "qb" which has very high compatibility with QuickBasic. We use the default "fb" dialect, which is the most C++-like dialect and removes a lot of QB syntax like GOSUB.