Workaround for not defining constructor for emplace()

Question

We have aggregate initialisation since C++11:

struct S {
    int count;
    double value;
};
S s{2, 3.0};

but in order for this work:

vector<S> v;
v.emplace_back(2, 3.0);

we need to manually write a constructor for no reason:

struct S {
    int count;
    double value;
    S(int count, double value): count(count), value(value) {}
};

We also need this constructor for optional(in_place, ...) and variant(in_place_type<>, ...) to work.

Is there a workaround for not writing this constructor manually?

It doesn't add any information just repeats what's already there, twice. Also the compiler is able to write it itself as it did it for the aggregate initialisation.

I was able to add new constructors to Egg's variant which allowed this but can we do something without modifying the container's source?

Edit: this question: Why doesn't emplace_back() use uniform initialization? is related but does not offer a workaround.

@kerrytazi That defeats the purpose of using `emplace_back`. Might as well use `push_back` at that point. — NathanOliver, Aug 14 '18 at 19:43
[This answer](https://stackoverflow.com/a/13845075/4342498) looks to have a clever work around — NathanOliver, Aug 14 '18 at 19:43
@SombreroChicken, thanks, I know the reasons, I'm looking for workaround. — tamas.kenez, Aug 14 '18 at 19:44
@NathanOliver thanks, a little heavy-weight but good to know. — tamas.kenez, Aug 14 '18 at 20:29

score 0 · Answer 1 · answered Aug 14 '18 at 20:32

0

You have at least one implicit constructor available.

std::vector<S> v;
v.emplace_back(S{2, 3.0});

(emplace_back and push_back are interchangeable in this context.)

answered Aug 14 '18 at 20:32

bipll

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Workaround for not defining constructor for emplace()

1 Answers1