C ++-私有数据成员

Question

If I have a class with private data members, for example, do I say that those data members are not accessible outside the class or they are not accessible outside the objects of that class?

Thanks.

Answer 1

Technically speaking, none of the above. You say, "Only entities that have private access to this class can access these variables."

This includes objects of that type , member functions of that type, friends of that type , member functions of friends of that type...

Actually, technically speaking, objects are incapable of accessing anything since they do not have behavior.

Answer 2

private意味着类的成员函数（以及任何嵌套类型）可以在给定类的任何实例的情况下访问那些数据成员。

Answer 3

If it is private, then (emphasis added):

its name can be used only by members and friends of the class in which it is declared.

-- Stroustup's "The C++ Programming Language", and one of the draft standards.

In C++, the data itself can always be accessed by other mechanisms. The goal is to impede accidental access, even if malicious access is still feasible.

Answer 4

They are not accessible outside the class's code (including derived classes); except for entities declared friend . As the class's code (the class member functions) are bound to the class (not the individual object), accessibility is evaluated at the class level.

class Foo
{
private:
    int secret;
    Foo * other;

public:
    explicit Foo(Foo* other_) : other(other_), secret(42) {}
    Foo() : other(0), secret(0) {}

    int Peek(void) { return secret; }
    int neighborPeek(void)
    {
        if (other)
            return other->secret; // this is OK, we're still inside the class
        else
            return -1;
}

int main()
{
    Foo aFoo, bFoo(&aFoo);
    std::cout << bFoo.neighborPeek(); // will dump aFoo's secret.

    return 0;
}