C++11 When clearing shared_ptr, should I use reset or set to nullptr?

Is there any real difference, or are there advantages/disadvantages to either approach?

The two alternatives are absolutely equivalent, in the sense that the second form (foo = nullptr) is defined in terms of the first one. Per Paragraph 20.7.1.2.3/8-10 of the C++11 Standard:

 unique_ptr& operator=(nullptr_t) noexcept;

8 Effects: reset().

9 Postcondition: get() == nullptr

10 Returns: *this.

Therefore, just choose the one which makes its intent clearest for you. Personally, I prefer:

foo = nullptr;

Because it makes it more evident that we want the pointer to be null. As a general advice, however, try to minimize the situations where you need to explicitly reset a smart pointer.

Besides, rather than using new:

std::shared_ptr<std::string> foo(new std::string("foo"));

Consider using std::make_shared() when possible:

auto foo = std::make_shared<std::string>("foo");