What's the idiomatic way to get a backwards inserter with a fallback to any inserter?

Question

I want to insert elements into a STL container (whose type is a template parameter). If the container allows for back_insertion, I want to use that, otherwise just any inserter.

I would like to avoid having to implement my own trait just for this, I'm pretty sure there must be someway I could say "instantiate a back_inserter if possible, and an inserter otherwise".

How do I do that?

score 3 · Answer 1 · edited Apr 28 '16 at 13:32

3

You could use a trailing return type with overload resolution to do something like:

namespace impl
{
   // preferred overload: if we can push_back a Container::value_type
   template <class Container>
   auto generic_inserter(Container& c, int)
       -> decltype(void(c.push_back(std::declval<typename Container::value_type>())),
                   std::back_inserter(c))
    {
        return std::back_inserter(c);
    }

    // fallback if we can't
    template <Container>
    auto generic_inserter(Container& c, ...)
    {
        return std::inserter(c, c.end());
    }
}

template <class Container>
auto generic_inserter(Container& c)
{
    return impl::generic_inserter(c, 0);
}

With C++11, simply replace the auto return types with decltype(body-of-function).

edited Apr 28 '16 at 13:32

Holt

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answered Apr 28 '16 at 12:27

Barry

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So nothing in the library itself? Hmm. Ok. – einpoklum Apr 28 '16 at 12:33
What is the purpose of the `void(...)` around `c.push_back`? – Holt Apr 28 '16 at 12:34
@Holt To avoid overloaded `operator,` – Barry Apr 28 '16 at 12:58
@Barry I don't get why this would add anything since `c.push_back()` already returns `void`? (Sorry if this is a dumb question... ) – Holt Apr 28 '16 at 13:12
@Holt General habit for writing trailing `decltype` with multiple expressions. – Barry Apr 28 '16 at 13:28
@Barry Okay, I would not have think of such a side case! – Holt Apr 28 '16 at 13:29

What's the idiomatic way to get a backwards inserter with a fallback to any inserter?

1 Answers1