On 22 January 2025 15:29:38 UTC, Dennis Gusev via Std-Proposals <std-proposals_at_[hidden]> wrote:
>If you look up "inheriting from STL containers", the top results from SO
>and reddit all mention it being a bad practice since standard containers
>aren't designed to be inherited from.
>
>Specifically, they dont have virtual destructors so there's a risk of UB
>from deleting through pointer to base.
>
>Also if you override any existing member functions and accidentally pass
>your derived class to a function expecting a regular STL container, the
>original base class member functions will get called instead(not to mention
>potential object slicing).
>
>What if C++ had a way to publicly inherit from a class and also ban base
>class conversions to avoid potential usage pitfalls with something like
>this:
>
>class Base
>
>{
>
> public:
>
> void foo()
>
> {
>
> std::println("Base foo");
>
> }
>
> void bar()
>
> {
>
> std::println("Base bar");
>
> }
>
>};
>
>class PublicDerived : public Base
>
>{
>
> public:
>
> void foo()
>
> {
>
> std::println("PublicDerived foo");
>
> }
>
>};
>
>class ComposedDerived : composed Base // new type of inheritance
>
>{
>
> public:
>
> void foo()
>
> {
>
> std::println("ComposedDerived foo");
>
> }
>
>};
>
>void testFoo(Base& val)
>
>{
>
> val.foo();
>
>}
>
>int main()
>
>{
>
> PublicDerived publicDerived;
>
> publicDerived.foo(); // prints "PublicDerived foo"
>
> publicDerived.bar(); // accessible, prints "Base bar"
>
> testFoo(publicDerived); // uh oh, prints "Base foo" even though we have
>our own foo
>
> ComposedDerived composedDerived;
>
> composedDerived.foo(); // prints "ComposedDerived foo"
>
> composedDerived.bar(); // still accessible, prints "Base bar"
>
> testFoo(composedDerived) // doesn't compile, conversion to base
>forbidden
>
> return 0;
>
>}
>
>Has anything like this ever been proposed?
>
>Seems like it would address a common use case by making it possible to
>extend behavior of classes safely even if they weren't designed to be
>inherited from(i.e. no public virtual or protected nonvirtual destructor)

Protected and public inheritance.

Are they not enough?