Understanding move constructor, std::move, and destructor
c++, c++11, destructor, move
Solution
If you move from one object to another object, you still have two objects in total. They both need to be destroyed. Perhaps `move` is a slightly misleading term, but it is not the object itself that is moved from one place to another (objects never actually move) - it's the contents of the object.
- No. As stated above, a move does not disappear away one of the objects. Both a copy and move from one object to another will have two objects involved. The difference is what effect they have on the objects. A copy will of course copy the members of one object to the other. A move, on the other hand, will move the members from one object to another - typically a faster operation.
- N/A
Problem
I am new to c++11 and wrote the following code to understand how `std::move` works: ``` #include <queue> #include <stdio.h> class X { public: X(int x) : x_(x) {} ~X() { printf("X(%d) has be released.\n", x_); } X(X&&) = default; X& operator = (X&&) = default; X(const X&) = delete; X& operator = (const X&) = delete; private: int x_; }; int main() { std::queue<X> xqueue; for (int x = 0; x < 5; ++x) { xqueue.push(std::move(X(x))); } return 0; } ``` However, it generates the following output, which indicates that the destructor of each `X(n)` has been called twice: ``` X(0) has be released. X(1) has be released. X(2) has be released. X(3) has be released. X(4) has be released. X(0) has be released. X(1) has be released. X(2) has be released. X(3) has be released. X(4) has be released. ``` I can imagine the second round of the output happens right at the end of `main()` function, and the first round probably happens in the loop when those intermediate `X`s ran out of scope. But, I thought the ownership of such intermediate `X`s will be perfectly transferred into the queue and their destructors should not be called during their ownership transfer. So my questions are: - When I saw an instance being deallocated twice, does that mean it performs copy instead of move? - If the above answer is yes, then how can I really avoid copying? Thank you,