A20-8-6

[martin.m.dowie]

What is the preferred method of assigning to a local member data of type std::shared_ptr<T>?

Given the code below, are the lines indicated as A20-8-6 non-compliant really so and why? We can understand C2 (types match exactly). We can understand C4 (types match exactly and the member data is of std::shared_ptr<T>&.

The class C1 we believe is non-compliant but we can't justify ourselves of the reason why.

Is the rule trying to enforce 3 e) from GoTW #91 (https://herbsutter.com/2013/06/05/gotw-9...arameters/)?

Code:

struct S {};

class C1 {
    std::shared_ptr<S> s;
public:
    C1(const std::shared_ptr<S>& aS) : s(aS) {} // A20-8-6 non-compliant
};

class C2 {
    std::shared_ptr<S> s;
public:
    C2(std::shared_ptr<S> aS) : s(aS) {} // OK
};

class C3 {
    std::shared_ptr<S> s;
public:
    C3(std::shared_ptr<S>& aS) : s(aS) {} // A20-8-6 non-compliant
};

class C4 {
    const std::shared_ptr<S>& s;
public:
    C4(const std::shared_ptr<S>& aS) : s(aS) {} // OK
};

void f () {
    auto s = std::make_shared<S>();
    C1 c1(std::make_shared<S>());
    C2 c2(std::make_shared<S>());
    C3 c3(s);
    C4 c4(std::make_shared<S>());
}