Check if multiple insertions into a set are successful

Let's write a helper that inserts an arbitrary number of variadic parameters into an std::set and returns if all the insertions are successful:

template <typename T, typename ... Ts>
bool insert_all(T &set, Ts ... ts)
{
 return (set.insert(ts).second && ...);
}

So, how does this work? The insert function of std::set has the following signature:

std::pair<iterator, bool> insert(const value_type& value);

The documentation says that when we try to insert an item, the insert function will return an iterator and a bool variable in a pair. The bool value is true if the insertion is successful. If it is successful, the iterator points to the new element in the set. Otherwise, the iterator points to the existing item, which would collide with the item to be inserted.

Our helper function accesses the .second field after insertion, which is just the bool variable that reflects success or fail. If all the insertions lead to true in all the return pairs, then all the insertions were successful. The fold expression combines all the insertion results with the && operator and returns the result.

We can use it like this:

std::set<int> my_set {1, 2, 3};

insert_all(my_set, 4, 5, 6); // Returns true
insert_all(my_set, 7, 8, 2); // Returns false, because the 2 collides

Note that if we try to insert, for example, three elements, but the second element can already not be inserted, the && ... fold will short-circuit and stop inserting all the other elements:

std::set<int> my_set {1, 2, 3};

insert_all(my_set, 4, 2, 5); // Returns false
// set contains {1, 2, 3, 4} now, without the 5!