C++ 底漆 5 版。 shared_ptr 和关联容器

Question

In C++ primer 5 ed. by Stanley lipmann chapter 15 OOP it is said:

 std::multiset<std::shared_ptr<Quote>, decltype(compare)*> items{compare};

The elements in our multiset are shared_ptrs and there is no less-than operator for shared_ptr. As a result, we must provide our own comparison operation to order the elements (§11.2.2, p. 425). Here, we define a private static member, named compare, that compares the isbns of the objects to which the shared_ptrs point. We initialize our multiset to use this comparison function through an in-class initializer (§7.3.1, p. 274):"

But If I try this:

// a class that doesn't define < operator
struct A
{
    int x = 0;
};


int main()
{
    std::shared_ptr<A> pa(make_shared<A>());
    std::shared_ptr<A> pb(make_shared<A>());

    cout << (pb < pa) << endl; // 0

}

• Why my code works although class A doesn't define less than operator?

• The thing is that after checking cppreference about class std::shared_ptr I've found out that it has overloaded relational operators?!

• I've also compiled the code against C++11 and still works fine!

• So I'd like someone to explain to me that paragraph in the book. Thank you!

Answer 1

Why my code works although class A doesn't define less than operator?

Because it is irrelevant what operators A have when the set doesn't contain elements of type A .

The set contains shared pointers.

The thing is that after checking cppreference about class std::shared_ptr I've found out that it has overloaded relational operators?!

Cppreference is correct.

So I'd like someone to explain to me that paragraph in the book.

The book is wrong.