访问说明符和性能

Question

Can changing the access specifier of a class member (say from private to protected) affect the performance (speed of execution) of a class? If so, please explain how?

Answer 1

No. If it does, I would consider your compiler to be broken.

Typically, compiled (non-debug) executables contain no information about the layout, access level, and even type of class members. These are simply not required to execute the program, so in usual "don't pay for what you don't use" fashion they are stripped out completely, and thus cannot have any impact on performance.

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In theory it could indirectly affect optimizer but I doubt it does in practice. – Slava

Fair point. In C++, access specifiers are just a sanity check performed when naming things. It is always performed last (after overload resolution, name lookup, etc), and only has two outcomes: either the named entity is accessible and all is well, or compilation halts. It cannot change the observable behaviour of a program at all. Thus, it would indeed be very strange that it have any impact on performance whatsoever.

Answer 2

Changing the access specifier of a data member can affect the layout .

If you have users of a (previously) standard layout type utilising offsetof or a common initial sequence, the sudden undefined behaviour of changing a member's access level allows the compiler large amounts of leeway in eliding "dead" code.

Even without that, you may have declared public data members in a particularly pessimal order, and private ing one allows a much more favorable layout. (or vice-versa)

However, you are free to re-order the declarations of members of your class, and that will have no other observable difference to your code. You should never change the access specifier of a member just for re-ordering purposes.