The evolution of window and class extra bytes in Windows

- The article discusses **window extra bytes** and **class extra bytes**, two old but still-supported Windows mechanisms that let developers attach custom data to a window or window class without creating a separate data structure. - **Raymond Chen** is a veteran Microsoft developer who writes "The Old New Thing" blog, a long-running deep-dive into Windows internals and history. - The post traces how these APIs evolved from 16-bit Windows (Windows 1.0/2.0 in the mid-1980s) through Win32 (Windows NT/95 onward) to modern 64-bit Windows. - Key tension: early Windows (16-bit) had severe memory constraints, so baking per-window storage into the window manager itself was a pragmatic shortcut. Over time, as memory became abundant and object-oriented programming took over, the mechanism became an awkward relic that coexists with modern approaches (like using `SetWindowLongPtr`/`GWLP_USERDATA` or C++ class wrappers). - Knowing this background helps make sense of why Windows still carries quirky, limited-size private storage on window objects — and why newer code should avoid it.