C++11 - emplacing a variable to any std container (vector, list, set, unordered_set)

Question

Trying to write a generic code to emplace a default constructed object into an std container and return an iterator to it.

with insert the solution is

template<typename C>
typename C::iterator insert(C& container)
{
    return container.insert(container.end(), typename C::value_type());
}

But emplace has no such common interface so I had to specialize it for 2 container types. My best attempt (that did not compile) was:

template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace_back())
{
    return container.emplace_back();
}

template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;
}

This causes: error: void value not ignored as it ought to be

and I don't understand why my decltype was deduced as void.

Used gcc 4.8.5 with -std=c++11

Edit:

reproduced by

int main() { std::vector v; std::vector::iterator iv = emplace(v);

    std::set<int> s;
    std::set<int>::iterator is = emplace(s);
    return 0;

}

Edit 2:

second attempt

template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;
}

template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace(container.end()))
{
    return container.emplace(container.end());
}

works for verctor, but set is ambiguous.

I cannot reproduce. What containers did you test with, and with which compiler? — Quentin, Apr 12 '18 at 12:44
error reproduced with: `int main() { std::vector v; std::vector::iterator i = emplace(v); return 0; }` — Shloim, Apr 12 '18 at 12:51

score 4 · Answer 1 · answered Apr 12 '18 at 12:51

4

Well, it took me longer than it should've.

emplace_back() only returns the emplaced object since C++17. Before then, it returns void, and even in C++17 the return type is a reference, not an iterator.

It looks like your specialization for emplace_back() should be modified to return container.back() instead.

answered Apr 12 '18 at 12:51

Quentin

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Thanks! Changed to `container.emplace(container.end())` – Shloim Apr 12 '18 at 13:05
But now the emplace for set is ambiguous. – Shloim Apr 12 '18 at 13:10
Posted a fix. Thanks for your help! – Shloim Apr 15 '18 at 11:50

score 0 · Accepted Answer · answered Apr 15 '18 at 11:50

Finally solved it thanks to @Quentin:

// For vector, list and anything push_backable
template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace_back(),container.rbegin())
{
    container.emplace_back();
    return container.rbegin();
}

// For set,unordered_set, and anything that emplace returns a pair.
template<typename C>
auto emplace(C& container)
    -> decltype(container.emplace().first)
{
    return container.emplace().first;
}

C++11 - emplacing a variable to any std container (vector, list, set, unordered_set)

2 Answers2