c++11 Why doesn't this move constructor work?

Question

I've written the code posted below. I was hoping to get to move the content of a vector between instances of LargeClass. The move constructor is being used, but instead of moving I get copies only.

Why doesn't the move semantics work as expected here?

Code:

#include <iostream> #include <vector> class LargeClass { public: explicit LargeClass (void): numbers(20, 10) { } LargeClass (const LargeClass &rhs): numbers(rhs.numbers) { std::cout << "Using LargeClass copy constructor" << '\n'; } LargeClass (const LargeClass &&rhs): numbers(std::move(rhs.numbers)) { std::cout << "Using LargeClass move constructor" << '\n'; } const int* getNumbersAddress(void) const { return (numbers.data()); } private: std::vector<int> numbers; }; int main() { LargeClass l1; std::cout << "l1 vector address: " << l1.getNumbersAddress() << '\n'; LargeClass l2(l1); std::cout << "l1 vector address: " << l1.getNumbersAddress() << '\n'; std::cout << "l2 vector address: " << l2.getNumbersAddress() << '\n'; LargeClass l3 = std::move(l2); std::cout << "l1 vector address: " << l1.getNumbersAddress() << '\n'; std::cout << "l2 vector address: " << l2.getNumbersAddress() << '\n'; std::cout << "l3 vector address: " << l3.getNumbersAddress() << '\n'; return 0; }

Possible output:

l1 vector address: 0x18ce010 Using LargeClass copy constructor l1 vector address: 0x18ce010 l2 vector address: 0x18ce070 Using LargeClass move constructor l1 vector address: 0x18ce010 l2 vector address: 0x18ce070 l3 vector address: 0x18ce0d0

It moves the content betwin adresses, but the addresses of containers remain the same no? — user1585121
– user1585121, Commented Jun 30, 2014 at 7:15
The syntax for move constructor should be LargeClass ( LargeClass &&rhs ) — P0W
– P0W, Commented Jun 30, 2014 at 7:15
@POW please post this as an answer. What a stupid mistake... Thanks! — Sebastian Kramer
– Sebastian Kramer, Commented Jun 30, 2014 at 7:17

P0W · Accepted Answer · 2014-06-30 07:39:22Z

The rvalue references don't make sense in their const forms because you want to modify them (you want to "move" them ). Objects created as const in C++ are in read-only memory, from which grabbing/modifying internal resources won't be possible.

The syntax for move constructor normally should be

class_name ( class_name && )

So use :

LargeClass ( LargeClass&& rhs )

Lightness Races in Orbit · Accepted Answer · 2014-06-30 10:25:24Z

Although a constructor taking const LargeClass&& is technically known as a move constructor (12.8/3) (which is baffling, but explicitly stated in [C++11: 12.8/3]), it's pretty bloomin' useless; it's self-evident that you cannot move from a const expression, because you cannot modify const things and moving implies altering the source! In particular, you cannot actually move from a const vector, which is why that does not occur here and why you see what you see.

A more useful move constructor looks like this:

LargeClass(LargeClass&& rhs)

Strictly by the book, it is a move constructor. Not a very useful one, but still.
And the move constructor is, in fact, invoked. It just doesn't do what the OP expects.

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