Visibility of private field of a template class from a template function-CodePudding

I have a template class

template<class T>
class MyClass
{
public:
    MyClass() { privateField = 0; };
    T getPrivateField() {
        return privateField;
    }
private:
    T privateField;
};

and a template function which takes an instance of MyClass as a parameter

template<class T>
T foo(MyClass<T> mc) {
    return mc.privateField;
}

I was confused by the fact that I can see a private field of MyClass in template function, but can't actually use it.
Here is an example of field visibility(screenshot)
Question:
Why can I see a private field of MyClass exactly in a template function and how can I disable it in my code(if it's possible)? Or it's just like a feature from Visual Studio?

CodePudding user response：

private will limit your code's access to a member.

It does not make parts of your code invisible to tools that might make observations about what you have written. It's not private in the sense that a personal password might be private.

CodePudding user response：

private does not mean that it is completely hidden from the outside, or that nobody outside of the class should be aware that it exists.

Consider this example:

#include <iostream>

struct foo {
    int x = 0;
};

struct bar : foo {
    private:
        int x = 0;
};

int main() {
    bar b;
    b.x = 0;
}

Now suppose, main was written by a user who only knows about the public parts of the involved classes. They would have no way to understand why they get the error message:

<source>: In function 'int main()':
<source>:15:7: error: 'int bar::x' is private within this context
   15 |     b.x = 0;
      |       ^
<source>:10:13: note: declared private here
   10 |         int x = 0;
      |             ^

Admittetly this is a made up example, and bar can be considered broken. The point is just that your IDE would not necessarily do you a favor if it would not show any private members. Their presence or absence can change the meaning of code that is written only against the public interface.

CodePudding user response：

Do not confuse visibility with access.

Consider how overload resolution behaves:

class X {
    private:
        void f(int value);
    public:
        void f(double value);
};

If we try to call X::f() with an int from outside:

int main() { 
    X obj;
    obj.f(1234);
}

We still get the error:

<source>:10:15: error: 'void X::f(int)' is private within this context
        obj.f(1234);

We definitely would not want main to call X::f(double) simply because it could not access the X::f(int) function. Otherwise, access could literally change the runtime meaning of your program.

C will

build an overload set from name lookup
perform resolution to pick the best match
verify access

In that order. That is, access is checked after the overload resolution has completed.