[fhd2]

> looking on example 1, I wonder why don't languages like C++ or D or Go just add a "pure" keyword for functions that don't modify the global environment or their arguments?

The function in example 1 is modifying a member variable, and there is indeed a keyword that requires functions not to modify the class they operate on: const. It's very powerful, and by a long shot my favourite feature of C++.

That said, the function in question actually has to modify a member variable.

[mjn]

The problem here isn't updateCachedWidth(), which indeed modifies a member variable, but deviceScaleFactor(), which doesn't. Since LLVM doesn't infer that deviceScaleFactor() leaves m_cachedWidth unmodified, it forces a reload of m_cachedWidth after the function call in case the value changed.

[RogerL]

It was an example. Replace deviceScaleFactor with some other function that modifies, say, m_foo, and you have the same problem. And no, const+mutable is not an answer(in my example), because the function does in fact modify the class in a way important to the caller.