emplace and default constructors

Question

Given the following code, I was surprised that try_emplace could not work out to use the default constructor demonstrated in the first line of the main function, instead complaining there is no matching function call to Element::Element(double, double). Have I misunderstood the way the compiler creates default constructors or the usage of try_emplace? I can of course get this code to work by defining an all parameter ctors for Element, but this seems redundant.

#include <string>
#include <map>

struct Element
{    
    double a;
    double b;
};

int main(int argc, char** argv)
{
    Element e {2.0, 3.0};

    std::map<std::string, Element> my_map;
    my_map.try_emplace("hello", 2.0, 3.0);

    return 0;
}

The compiler doesn't generate c'tor s with parameters, and `try_emplace` will only try to call a c'tor (no aggregate initialisation). — George, Aug 21 '18 at 10:49
But then where does the definition for `Element e{2.0, 3.0}` come from? — Madden, Aug 21 '18 at 10:51
@Madden Element is an aggregate (a POD actually), for which brace initialization is supported, even without any constructor. I think that this is a deficiency in the standard, and it should have had a special code in emplace_back for aggregates. This would have been more consistent and less surprising. — Michael Veksler, Aug 21 '18 at 11:06

score 5 · Answer 1 · answered Aug 21 '18 at 10:58

5

Alternatively, you can opt to not define any user-defined ctors in Element but rather use those that are still defined no matter what (unless explicitly deleted):

    my_map.try_emplace("hello", Element{2.0, 3.0});

answered Aug 21 '18 at 10:58

bipll

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Will this method entail a copy of the Element object? – Madden Aug 21 '18 at 11:02
@Madden - Yes. But seeing as this type is a simple trivially copiable aggregate, it likely won't do any worse than forwarding two doubles. Don't optimize prematurely. – StoryTeller - Unslander Monica Aug 21 '18 at 11:05
Sure, but this was more of a simple example - in practice I intend to do a lot of emplacing and with much more complex/larger objects. I will define specific constructors to make sure I avoid the copy – Madden Aug 21 '18 at 11:10
@Madden Such templated functions usually forward their arguments, so if you have proper overloads accepting rvalue references, they should be called. – bipll Aug 21 '18 at 11:13

score 4 · Answer 2 · answered Aug 21 '18 at 10:52

This is due to the fact that emplace methods attempt to instantiate elements with parentheses, not by aggregate initialization. You can verify that

Element e(2.0, 3.0);

fails to compile, because there is no such constructor (Element e{2.0, 3.0} bypasses such a constructor call). But this is what emplace tries to do. To fix this, you can add an appropriate constructor:

struct Element
{
    Element(double a, double b) : a(a), b(b) {}

    double a;
    double b;
};

emplace and default constructors

2 Answers2