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Platforms.md

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+We use **cmake**, so a wide-variety of platforms *might* be supported.
+
+## Travis CI
+We run builds and tests in Travis for
+- **linux**
+- Mac **osx** (*coming soon*!)
+
+For *each* os, we try both:
+- **gcc**
+- **clang**
+
+See [Travis docs](http://docs.travis-ci.com/user/ci-environment/) for exact versions. See `.travis.yml` for details of our build matrix.
+
+## Appveyor
+We run builds and tests in appveyor for
+- Windows 32-bit
+- Windows 64-bit
+
+See [Appveyor docs](http://www.appveyor.com/docs/installed-software) for exact versions. It currently uses VisualStudio 2013, but we are told that [VS 2012 works fine](https://github.com/open-source-parsers/jsoncpp/issues/246) too.
+
+## Others
+Lots of older compilers and other platform variants might work too. If you find something broken, please let us know.
+
+## C++ version
+Different branches of our code expect different versions of C++. Travis/Appveyor simple runs the latest C++ compiler possible, and our own `#ifdef` macros handle the differences. The `master` branch expects **C++11**. The `0.y.z` branch can use something earlier than even C++0x; for the benefit of Android, it does not even require C++11 *libraries*. (Hopefully we can relax that constraint someday.)