copy vs std::move for ints

Question

• What\'s difference between default copy and std::move in that example?
• After move the object is there any dependence between new and old ones?

Answer 1

In this example, there is no difference. We will end up with 3 ints with value 100. There could definitely be a difference with different types though. For instance, let's consider something like vector:

std::vector a = {1, 2, 3, 4, 5}; // a has size 5
auto a_copy = a;                      // copy a. now we have two vectors of size 5
auto a_move = std::move(a);           // *move* a into a_move

The last variable, a_move, takes ownership of a's internal pointers. So what we end up with is a_move is a vector of size 5, but a is now empty. The move is much more efficient than a copy (imagine if it was a vector of 1000 strings instead - a_copy would involve allocating a 1000-string buffer and copying 1000 strings, but a_move just assigns a couple pointers).

For some other types, one might be invalid:

std::unique_ptr a{new int 42};
auto a_copy = a;            // error
auto a_move = std::move(a); // OK, now a_move owns 42, but a points to nothing

For many types, there's no difference though:

std::array a;
auto a_copy = a;            // copy 100 ints
auto a_move = std::move(a); // also copy 100 ints, no special move ctor

More generally:

T a;
auto a_copy = a;            // calls T(const T& ), the copy constructor
auto a_move = std::move(a); // calls T(T&& ), the move constructor