Re: [boost] Report from Mont Tremblant C++ Committee meeting

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-10-15 09:29:05

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