Qt & OpenGL: How do I force OpenGL 2.1?

I'm developing an application that makes use of Qt and OpenGL, using Qt Creator and QGLWidget subclassing.

My application has a user base that has a higher than average proportion of older hardware, which is why I need it to run on machines with graphics cards supporting OpenGL 2.1 only - or, in other words, I cannot rely on anything newer than 2.1 being present.

I am worried about unknowingly using OpenGL functionality that was introduced after 2.1. Is there any way I can configure OpenGL to "only" support 2.1, so that I would get a runtime error if I do something I shouldn't be doing? Or, failing that, what is the best practice to ensure compatibili开发者_JAVA百科ty?

The only thing you need to worry about is not creating a OpenGL-3 core profile context, and only to use functions found in the OpenGL-2.1 specification.

Since creating a OpenGL-3 core context requires you to jump some hoops, you're not running into problems there. The system may give you something newer than OpenGL-2.1 but as long as you don't use any functionality not found in the 2.1 specification document you're fine. You'll have to use the extension mechanism, to actually get the functionality on Windows; OpenGL-2.1 is technically a list of extensions made official functionality, so carefully read the Appendix of the specification, where the functionality that formerly were extensions are explicitly mentioned.