default access modifier in C++

Question

I have an interface in C++ that looks something like this:

// A.h
#pragma once

class A
{
public:
    //Some declarations.    
private:    
    //Some declarations.
protected:
    //Some declarations.
};

The specific form is not important. Since this is an interface, there will be a class B that inherits from A. In the header file for class B I have:

// B.h
#pragma once

class B : A
{
public:
    //Some declarations.    
private:    
    //Some declarations.
protected:
    //Some declarations.
};

My concern is that I tend to use class B : A instead of class B : public A, just my bad memory.

So far I have had no issues with this, since it's a small enough project. But will forgetting the public keyword affect my project in any sense?

Or more succinctly, I know how access modifiers work but, what does class B : A default to?

Duplicate? https://stackoverflow.com/questions/860339/difference-between-private-public-and-protected-inheritance — , Mar 12 '21 at 17:06
Hi, I already took a look at that, it does not address the default modifier. — , Mar 12 '21 at 17:08
It doesn't? `class D : private A // 'private' is default for classes` — , Mar 12 '21 at 17:09
You can find the details here: https://en.cppreference.com/w/cpp/language/access — Jesper Juhl, Apr 17 '23 at 07:20

score 3 · Accepted Answer · answered Mar 12 '21 at 17:05

3

The ONLY difference between struct and class is that in a struct, everything is public until declared otherwise, and in a class, everything is private until declared otherwise. That includes inheritance. So class B : A will default to private inheritance, and struct B : A will default to public inheritance.

answered Mar 12 '21 at 17:05

Mooing Duck

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@rustyx Sure, but answering the question as asked without mentioning that `struct` works the opposite way would have been misleading. – Mooing Duck Mar 12 '21 at 17:33

score 2 · Answer 2 · answered Mar 12 '21 at 17:17

What does class B : A default to?

class B : private A { /*...*/ }

But will forgetting the public keyword affect my project in any sense?

Yes. Don't forget it.

Consider the following:

// A.h
class A {

public:
   void f(){}  
};

// B.h
class B : A {};

int main() {

   B b;
   b.f(); // f is inaccessible because of private inheritance
}

default access modifier in C++

2 Answers2