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Could C++ have not obviated the pimpl idiom?

Asked 2008-11-06T15:56:44.270
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As I understand, the pimpl idiom is exists only because C++ forces you to place all the private class members in the header. If the header were to contain only the public interface, theoretically, any change in class implementation would not have necessitated a recompile for the rest of the program.

What I want to know is why C++ is not designed to allow such a convenience. Why does it demand at all for the private parts of a class to be openly displayed in the header (no pun intended)?

c++ language-design compiler-theory pimpl-idiom
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Someone will have a much more verbose answer than I, but the quick response is two-fold: the compiler needs to know all the members of a struct to determine the storage space requirements, and the compiler needs to know the ordering of those members to generate offsets in a deterministic way.

The language is already fairly complicated; I think a mechanism to split the definitions of structured data across the code would be a bit of a calamity.

Typically, I've always seen policy classes used to define implementation behavior in a Pimpl-manner. I think there are some added benefits of using a policy pattern -- easier to interchange implementations, can easily combine multiple partial implementations into a single unit which allow you to break up the implementation code into functional, reusable units, etc.

answered 2008-11-06T16:02:59.497

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