Re: std::pair<reference,reference>

[Howard Hinnant <hhinnant at apple dot com>]

On May 18, 2006, at 12:38 PM, Benjamin Kosnik wrote:

> > No I mean this:
> >
> > #include <utility>
> >
> > int main()
> > {
> >  using namespace std;
> >
> >  int i = 6;
> >  int j = 7;
> >  int& r1 = i;
> >  int& r2 = j;
> >  pair<int&, int&> p = make_pair(r1, r2);
> > }
>
> Sure, your initial suggestion seems like a winner.

You might take a look at how tuple is done (which already has this  
functionality).

>       pair(const _T1& __a, const _T2& __b)
>       : first(__a), second(__b) { }

Besides the reference-to-reference problem this will allow you to  
bind a non-const reference to a temporary (which is bad).

In a nutshell, if you've got a reference type, you need to accept it  
by value in an argument list:

template <class T>
struct param
{
    typedef const T& type;
};

template <class T>
struct param<T&>
{
    typedef T& type;
};

template <class T1, class T2>
struct pair
{
    pair(typename param<T1>::type a, ...);
};

pair<int&, int&> p(1, 2);  // compile time error (which is good)
int i;
pair<int&, int&> p(i, i);  // ok

Also:

>       typedef _T1 first_type;    ///<  @c first_type is the first  
> bound

should be:

typedef _T1& first_type;

You don't want to tell people you've got an int when you really have  
an int&.

-Howard