问题
Besides the normal explenation of being visible or not to derived classes, is their any other difference?
If you make it more visible, is it taking more or less memory, does it slow thing down or...?
回答1:
Apart from the accessibility of members outside or to the derived classes, access specifiers might affect the object layout.
Quoting from my other answer:
Usually, memory address for data members increases in the order they're defined in the class . But this order may be disrupted at any place where the access-specifiers (private, protected, public) are encountered. This has been discussed in great detail in Inside the C++ Object Model by Lippman.
An excerpt from C/C++ Users Journal,
The compiler isn't allowed to do this rearrangement itself, though. The standard requires that all data that's in the same public:, protected:, or private: must be laid out in that order by the compiler. If you intersperse your data with access specifiers, though, the compiler is allowed to rearrange the access-specifier-delimited blocks of data to improve the layout, which is why some people like putting an access specifier in front of every data member.
Interesting, isn't it?
回答2:
From n3225, 9.2 [class.mem] note 15
Nonstatic data members of a (non-union) class with the same access control (Clause 11) are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members with different access control is unspecified (11).
This means that given the following declaration:
class Foo {
public: int a;
private: int b;
public: int c;
private: int d;
};
Only the following assertions are enforced by the standard:
Foo foo;
assert(&foo.a < &foo.c);
assert(&foo.b < &foo.d);
@Nawaz's citation can be interpreted as giving 4 blocks that can be freely intermixed, but this is not the case. The declaration of Foo is perfectly equivalent to:
class Foo' {
public: int a,c;
private: int b,d;
};
Indeed the compiler completely ignores (for this purpose) whether a specifier appeared once or multiple times, and specifying it each time is spurious and at best slows the compilation down because of the extra parsing. For a human reader, it might be clearer... but this is highly subjective.
来源：https://stackoverflow.com/questions/4978302/besides-accessibility-what-else-access-specifiers-effects