C++ primer 5 ed. shared_ptr and associative containers

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!

_there is no less-than operator for shared_ptr_... are you sure? — Mike Vine, Aug 06 '20 at 23:32
https://en.cppreference.com/w/cpp/memory/shared_ptr/operator_cmp — Mike Vine, Aug 06 '20 at 23:32
Just checked that book on Amazon "Rewritten for the new C++11 Standard". Yikes - you ought to get a more uptodate book. See https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Mike Vine, Aug 06 '20 at 23:40
It's working because it's not comparing the objects, it's comparing the pointers to the objects. From the link above Note that the comparison operators for shared_ptr simply compare pointer values; the actual objects pointed to are not compared. Having operator< defined for shared_ptr allows shared_ptrs to be used as keys in associative containers, like std::map and std::set. — Durstann, Aug 06 '20 at 23:40

score 2 · Accepted Answer · answered Aug 06 '20 at 23:49

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.

C++ primer 5 ed. shared_ptr and associative containers

1 Answers1