Access modifiers

Names of keywords

C++ uses the three modifiers called public, protected, and private. C# has the modifiers public, protected ,internal, private, protected internal, private protected, and file. Java has public, package, protected, and private; package is the default, used if no other access modifier keyword is specified. The meaning of these modifiers may differ from one language to another. A comparison of the keywords, ordered from the most restrictive to the most open, and their meaning in these three languages follows. Their visibility ranges from the same class to the package where the class is defined to a general access permission. Below, the maximal access is written into the table.

In Swift, there are five different access levels relative to both the source file in which the entity is defined and the module containing that source file.

Example

C++

import std;

// default access modifier inside struct is public
// default access modifier inside class is private
struct Base {
private:
    int x;
protected:
    // Base::m, Base::n are protected
    int m;
    int n;
public:
    void set_n(int v) noexcept { 
        n = v; 
    }

    void f() { 
        std::println("calling Base::f()");
    }
};
 
// Derived extends Base with public inheritance
struct Derived : public Base {
private:
    using Base::f; // Derived::f is private
public:
    using Base::m; // Derived::m is public

    // Base::n is accessible here, but not outside
    [[nodiscard]]
    int get_n() const noexcept { 
        return n; 
    }

    // ERROR, Base::x is inaccessible here
    // int get_x() const noexcept { 
    //     return x; 
    // }
};
 
int main(int argc, char* argv[]) {
    Derived d;

    // d.x = 2; // ERROR, private
    // d.n = 2; // ERROR, protected
    d.m = 2; // protected Base::m is accessible as Derived::m

    d.set_n(2); // calls Base::set_n(int)
    std::println("{}", d.get_n()); // output: 2

    // d.f(); // ERROR, Base::f is inaccessible as Derived::f

    Base& b = d; // b references d and "views" it as being type Base

    // b.x = 3; // ERROR, private
    // b.n = 3; // ERROR, protected
    // b.m = 3; // ERROR, Base::m is protected

    b.set_n(3); // calls Base::set_n(int)
    // std::println("{}", b.get_n()); // ERROR, 'struct Base' has no member named 'get_n'

    b.f(); // calls Base::f()
    return 0;
}

References

Notes

Bibliography

References

1. Wagner, Bill. "Access Modifiers - C# Programming Guide".
2. "Access specifiers".
3. (2022-09-27). "Access Modifiers (C# Reference)". [[Microsoft]].
4. "Access Control — The Swift Programming Language (Swift 5.3)".