Move Semantics & Rvalue References

Copying a large object (a vector with a million elements, say) is expensive — move semantics (C++11) let ownership of that object's internal data transfer instead, leaving the source in a valid-but-unspecified "moved-from" state, for the cost of just copying a few pointers.

std::vector<int> makeData() {
    std::vector<int> result = {1, 2, 3, 4, 5};
    return result; // moved out, not copied, on return
}

std::vector<int> a = makeData();      // move, not copy
std::vector<int> b = std::move(a);    // explicit move — a is now empty

std::move doesn't itself move anything — it's a cast that marks a value as safe to move from, which the receiving constructor or assignment operator then actually acts on.

Common mistakes

• Using a variable after std::moveing it, expecting its old contents to still be there — a moved-from object is only guaranteed to be in some valid state, not its original one.