Private variables are a way to hide complexity and implementation details to the user of a class. This is a rather nice feature. But I do not understand why in c++ we need to put them in the header of a class. I see some annoying downsides to this:

• it clutters the header from the user
• it force recompilation of all client libraries whenever the internals are modified

Is there a conceptual reason behind this requirement? Is it only to ease the work of the compiler?