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by jchb 2774 days ago
> Not sure why we need to exclude pointers?

Sometimes you need to. You cannot entirely stack allocate an object that uses PIMPL. Also you cannot allocate an array of such objects compactly in memory.

On the other hand, if you want to be able to evolve the class member variables but still maintain a stable ABI, you need to hide the memory layout, for example with PIMPL. But this is a C++ limitation. For example Objective-C* (and also soon Swift) allows modifying the class layout, adding properties etc, without changing the ABI.

https://en.wikipedia.org/wiki/Objective-C#Non-fragile_instan...
3 comments
nchie 2774 days ago
> Sometimes you need to. You cannot entirely stack allocate an object that uses PIMPL.

You actually can, if I'm not misunderstanding: http://www.gotw.ca/gotw/028.htm

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beached_whale 2774 days ago
They must have some level of indirection in the resulting code to accomplish that like virtual inheritance. There is a price and this can be done in C++ too, but you have to opt into the cost.
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jcelerier 2774 days ago
> Under the modern runtime, an extra layer of indirection is added to instance variable access,

so it's just syntax sugar for PIMPL.

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jchb 2774 days ago
Actually not, it works a bit differently. In Objective-C 1, there was no indirection and one had to explicitly use the equivalent of PIMPL to hide private members from the header or avoid the fragile base class problem.

In Objective-C 2 the object meta-data contains a table of instance variable offsets. The dynamic linker can modify this table at load time so you can freely add both instance variables and methods to new revisions of a class.

So what is the deal? Well, when the holder object itself is heap allocated, pimpl is inefficient because every access will require dereferencing two pointers. Also you cannot put protected or virtual members in the internal pimpl class (then there would be no point to have those in the first place).

That being said, it is not like Objective-C is some pinnacle of performance - you cannot allocate objects on the heap, and the compiler doesn't perform any devirtualization. So for performance critical code you have to drop down to C or...C++ :)

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