CODEWITHSUNDEEP
c++move-semantics
VladG
VladG

Reputation: 143

Does element type in std::vector need to be movable to allow move semantics on return by value?

Does MyType have to be "movable" for the compiler to be able to apply move semantics when returning v from foo()?

std::vector<MyType> foo() {
    std::vector<MyType> v;
    // populate v
    return v;
}

Upvotes: 1

Views: 81

Answers (1)

NathanOliver
NathanOliver

Reputation: 180710

No, vector does not need your type to be moveable in order for the vector itself to be moveable. Essentially a vector is

template <typename T>
class vector
{
private:
    T* start;
    T* end;
    T* capcity_end;
public:
    ...
    vector(vector&& old) : start(old.start), end(old.end), capcity_end(old.capcity_end) {
        old.start = old.end = old.capcity_end = nullptr;
    }
    ...
};

And when the vector moves, it just copies the pointer values into the new vector, and sets to pointers to nullptr in the old vector so that they do not get cleaned up by the destructor. With this, T doesn't even need to be copyable, as no elements are being copied/moved, it's just pointers getting swapped.

Upvotes: 3

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