Is the compiler allowed to optimise out private data members?

Question

If the compiler can prove that a (private) member of a class is never used, including by potential friends, does the standard allow the compiler to remove this member from the memory footprint of the class?

It is self-evident that this not possible for protected or public members at compile time, but there could be circumstances where it is possible regarding private data members for such a proof to be constructed.

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Related questions:

• Behind the scenes of public, private and protected (sparked this question)
• Is C++ compiler allowed to optimize out unreferenced local objects (about automatic objects)
• Will a static variable always use up memory? (about static objects)

Answer 1

Possible in theory (along with unused public members), but not with the kind of compiler ecosystem we're used to (targeting a fixed ABI that can link separately-compiled code). Removing unused members could only be done with whole-program optimization that forbids separate libraries¹.

Other compilation units might need to agree on sizeof(foo), but that wouldn't be something you could derive from a .h if it depended on verifying that no implementation of a member function's behaviour depended on any private members.

Remember C++ only really specifies one program, not a way to do libraries. The language ISO C++ specifies is compatible with the style of implementation we're used to (of course), but implementations that take all the .cpp and .h files at once and produce a single self-contained non-extensible executable are possible.

If you constrain the implementation enough (no fixed ABI), aggressive whole-program application of the as-if rule becomes possible.

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Footnote 1: I was going to add "or exports the size information somehow to other code being compiled" as a way to allow libraries, if the compiler could already see definitions for every member function declared in the class. But @PasserBy's answer points out that a separately-compiled library could be the thing that used the declared private members in ways that ultimately produce externally-visible side effects (like I/O). So we'd have to fully rule them out.

Given that, public and private members are equivalent for the purposes of such an optimization.

Answer 2

If the compiler can prove that a (private) member of a class is never used

The compiler cannot prove that, because private members can be used in other compilation units. Concretely, this is possible in the context of a pointer to member in a template argument according to [temp.spec]/6 of the standard, as originally described by Johannes Schaub.

So, in summary: no, the compiler must not optimise out private data members any more than public or protected members (subject to the as-if rule).