Does using .reset() on a std::shared_ptr delete all instances

When you use .reset(), you are eliminating one owner of the pointer, but all of the other owners are still around. Here is an example:

#include <memory>
#include <cstdio>

class Test { public: ~Test() { std::puts("Test destroyed."); } };

int main()
{
    std::shared_ptr<Test> p = std::make_shared<Test>();
    std::shared_ptr<Test> q = p;
    std::puts("p.reset()...");
    p.reset();
    std::puts("q.reset()...");
    q.reset();
    std::puts("done");
    return 0;
}

The program output:

p.reset()...
q.reset()...
Test destroyed.
done

Note that p and q are both owners of the object, and once both p and q are reset, then the instance is destroyed.

No.

The whole purpose of shared_ptr is that you cannot delete it from one place if someone is using it in another. shared_ptr::reset() just decreases use_count by one and replaces its object by nullptr.

The .reset() method only applies to the object it's called upon.

It just replaces the pointer that variable is holding.