Does std::vector.push_back(std::move(foo)) make sense?
c++11, move-semantics, stl
Solution
Class `vector` has two `push_back` implementations:
void push_back( const T& value );
void push_back( T&& value );
The first one does copy of the element given.
The second tries to "move" it by calling element's move constructor (if it's defined).
Using `move` forces to pick the second implementation which is supposed to reuse the value rather than just copying one.
In this particular case this is what gonna happen:
- Vector `bar` is allocated on the stack, but its elements (42) are allocated on the heap.
- When you call `foo.push_back(...)`, `foo` allocates on the heap a new vector, which is gonna be `bar`s copy. Let's call it `baz` :) Depending on which `push_back` implementation is called the following will happen then:
- `void push_back( const T& value );`: in this case all `bar`'s elements will be copyed to `baz` as well.
- `void push_back( T&& value );` in this case `baz` will receive a pointer to `bar`'s elements, so no copy operations are performed. But it is crucial for understanding that `bar` will be deprived of its elemets (now `baz` owns them), so `bar` shoun't be used after `move`.
It isn't that important what kind of the elements are (plain ints or `pcl::PointXYZ`), since only the first vector has allocated the memory for the elements, and the pointer to that memory is the only thing that is copyed during the `move` call.
Problem
I have come across this in some code (details eliminated for clarity): ``` std::vector<std::vector<int>> foo; { std::vector<int> bar = {42}; foo.push_back(std::move(bar)); // Hmmm... } // Indicate `bar` is no longer needed. ``` The `std::move` looks unnecessary to me, but is it? Is the behaviour any different from just `foo.push_back(bar);`? What if instead of an `int` the element is a class such as `pcl::PointXYZ` as it is in my actual code? UPDATE: I have altered the code to more explicitly indicate that `bar` is not used after the `std::move`, so there is no illegal access, etc, risk.