On: Google C++ Style Guide

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It's also used by Apple in all of their Cocoa and CoreFoundation code. An example from CoreFoundation would be their constant CFAllocator references:

    const CFAllocatorRef kCFAllocatorDefault;
    const CFAllocatorRef kCFAllocatorSystemDefault;
    const CFAllocatorRef kCFAllocatorMalloc;
    const CFAllocatorRef kCFAllocatorMallocZone;
    const CFAllocatorRef kCFAllocatorNull;
    const CFAllocatorRef kCFAllocatorUseContext;