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### 561. inserter overly generic

Section: 25.5.2.4.2 [inserter] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The declaration of `std::inserter` is:

>     template <class Container, class Iterator>
>     insert_iterator<Container>
>     inserter(Container& x, Iterator i);

The template parameter `Iterator` in this function is completely unrelated to the template parameter `Container` when it doesn't need to be. This causes the code to be overly generic. That is, any type at all can be deduced as `Iterator`, whether or not it makes sense. Now the same is true of `Container`. However, for every free (unconstrained) template parameter one has in a signature, the opportunity for a mistaken binding grows geometrically.

It would be much better if `inserter` had the following signature instead:

>     template <class Container>
>     insert_iterator<Container>
>     inserter(Container& x, typename Container::iterator i);

Now there is only one free template parameter. And the second argument to `inserter` must be implicitly convertible to the container's iterator, else the call will not be a viable overload (allowing other functions in the overload set to take precedence). Furthermore, the first parameter must have a nested type named `iterator`, or again the binding to `std::inserter` is not viable. Contrast this with the current situation where any type can bind to `Container` or `Iterator` and those types need not be anything closely related to containers or iterators.

This can adversely impact well written code. Consider:

>     #include <iterator>
>     #include <string>

>     namespace my
>     {

>     template <class String>
>     struct my_type {};

>     struct my_container
>     {
>     template <class String>
>     void push_back(const my_type<String>&);
>     };

>     template <class String>
>     void inserter(const my_type<String>& m, my_container& c) {c.push_back(m);}

>     }  // my

>     int main()
>     {
>         my::my_container c;
>         my::my_type<std::string> m;
>         inserter(m, c);
>     }

Today this code fails because the call to `inserter` binds to `std::inserter` instead of to `my::inserter`. However with the proposed change `std::inserter` will no longer be a viable function which leaves only `my::inserter` in the overload resolution set. Everything works as the client intends.

To make matters a little more insidious, the above example works today if you simply change the first argument to an rvalue:

>         inserter(my::my_type(), c);

It will also work if instantiated with some string type other than `std::string` (or any other `std` type). It will also work if `<iterator>` happens to not get included.

And it will fail again for such inocuous reaons as `my_type` or `my_container` privately deriving from any `std` type.

It seems unfortunate that such simple changes in the client's code can result in such radically differing behavior.

Proposed resolution:

Change 24.2:

> 24.2 Header `<iterator>` synopsis

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.5:

> 24.4.2.5 Class template `insert_iterator`

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.6.5:

> 24.4.2.6.5 `inserter`

>     template <class Container, class Inserter>
>        insert_iterator<Container> inserter(Container& x, Inserter typename Container::iterator i);

> > -1- Returns: `insert_iterator<Container>(x,typename Container::iterator(i))`.

[ Kona (2007): This issue will probably be addressed as a part of the concepts overhaul of the library anyway, but the proposed resolution is correct in the absence of concepts. Proposed Disposition: Ready ]