On Oct 14, 2005, at 11:54 PM, Joel de Guzman wrote:
Interesting point! Now consider this:
int a = 1; int b = 2; tuple<int&> x(a); tuple<int&> y(b); swap(x, y);
What happens after the swap? The same thing happens!
before swap a: 1 b: 2 x: 1 y: 2 swap a: 2 b: 2 x: 2 y: 2
It is on my to-do list to propose a specialized swap for tuple, as I just did for pair: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/ n1856.html#20.2.3%20-%20Pairs The effect of the general swap on both pair and tuple are disastrous in the context of move semantics. Consider: typedef pair<int, vector<int> > Pair; Pair x, y; ... swap(x, y); If the general swap is used, this is an O(N) operation, but if instead pair does: swap(x.first, y.first); swap(x.second, y.second); then things just work, O(1). tuple should (will) have an analogous swap (as does tr1::array already - another "tuple like" type). This is already implemented in CodeWarrior Pro 10 (beta). The following: #include <tuple> #include <tupleio> #include <iostream> int main() { int a = 1; int b = 2; std::tr1::tuple<int&> x(a); std::tr1::tuple<int&> y(b); std::cout << "a: " << a << '\n'; std::cout << "b: " << b << '\n'; std::cout << "x: " << x << '\n'; std::cout << "y: " << y << '\n'; swap(x, y); std::cout << "swap(x, y)\n"; std::cout << "a: " << a << '\n'; std::cout << "b: " << b << '\n'; std::cout << "x: " << x << '\n'; std::cout << "y: " << y << '\n'; } Outputs: a: 1 b: 2 x: (1) y: (2) swap(x, y) a: 2 b: 1 x: (2) y: (1) So I'm not weighing in on either side of the optional<T&> debate at the moment. All I'm saying is that any argument based on the current behavior of swapping tuple<T&> is going to be false tomorrow (if I can at all influence the committee - and I feel very strongly about this one). -Howard