Can you use a constructor as a UnaryOperator?

Question

If I have a class with a unary constructor, can I somehow use that constructor as a UnaryOperator in an algorithm? i.e.:

#include <algorithm>
#include <vector>

class Thing
{
  public:
    Thing(int i)
    : m_i(i)
    {}
    
    int i() {
        return m_i;
    };
    
  private:
    int m_i;
};

int main() {
    std::vector<int> numbers{0,4,-13,99};
    std::vector<Thing> things;
    
    std::transform(numbers.begin(), numbers.end(), std::back_inserter(things), Thing);
}

the above code doesn't work, the code below works but I'm wondering if there's a better way:

#include <algorithm>
#include <vector>

class Thing
{
  public:
    Thing(int i)
    : m_i(i)
    {}

    int i() {
        return m_i;
    };

  private:
    int m_i;
};

int main() {
    std::vector<int> numbers{0,4,-13,99};
    std::vector<Thing> things;

    auto make_thing = [](auto&& i){return Thing(i);};

    std::transform(numbers.begin(), numbers.end(), std::back_inserter(things), make_thing);
}

[possible duplicate](https://stackoverflow.com/questions/44491995/using-class-constructor-as-callable-object) — Drew Dormann, Jun 01 '22 at 15:28
You cannot refer to or call a constructor at all (with some exceptions such as `using` declarations). In `Thing(i)`, `Thing` is not the constructor, but the type. It only calls the constructor indirectly. — user17732522, Jun 01 '22 at 16:35

score 2 · Accepted Answer · answered Jun 01 '22 at 15:33

For the first code sample just use a lambda:

    std::transform(numbers.begin(), numbers.end(), std::back_inserter(things), [](int i){ return Thing(i);});

And this is the best way for what your code does. Everybody will understand it.

Constructor can't be used as an UnaryOperator, because it does not return a value.

Can you use a constructor as a UnaryOperator?

1 Answers1