How to write proper std::initializer_list constructor

Question

Consider following code:

#include <iostream>
#include <vector>

struct C {
    std::vector<int> a;
    std::string b;
    bool c;
};

void printC(const C &c) {
    // ...
}

int main() {
    printC({
        { 1, 2, 3 },
        "ehlo",
        false
    });
}

This works, because compiler can generate proper constructor for me. But if I change struct C to this:

struct C {
    std::vector<int> a;
    std::string b;
    bool c;

    C() {
        c = false;
    }
};

The printC call stops working because compiler stops generating appropriate constructor. I've tried to write myself a constructor using std::initializer_list but failed.

So the question is - How to write constructor that will make the above code compile and work again?

Don't forget to #include <string>!

Lightness Races in Orbit
– Lightness Races in Orbit

2018-12-20 11:37:12 +00:00
Commented Dec 20, 2018 at 11:37 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Dec 20, 2018 at 11:37

StoryTeller - Unslander Monica · Accepted Answer · 2018-12-20 11:34:03Z

6

I've tried to write myself a constructor using std::initializer_list but failed.

You don't need one. You just need a c'tor taking a vector, string and boolean:

C(std::vector<int> a, std::string b, bool c) 
  : a(std::move(a))
  , b(std::move(b))
  , c(c) {
}

Your code should now be well-formed again. Though now it incurs two move operations, while the original aggregate version could have initialized the elements of your object directly. It's something worth considering.

edited Dec 20, 2018 at 11:34

answered Dec 20, 2018 at 8:04

StoryTeller - Unslander Monica

172k22 gold badges401 silver badges481 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Sebastian Redl Over a year ago

Eh, that's gonna get inlined. If you the code is really really performance-sensitive, you might want to look at the generated assembly to confirm, but in general, it is not something I would consider.

doomista Over a year ago

Context where I came accross this problem fortunately doesn't care about performance much. This is the correct answer, I originally had struct S nested in unordered map and didn't realize that not everything is initializer list.

StoryTeller - Unslander Monica Over a year ago

@doomista - You aren't gonna see a performance hit even with the moves. Vector and string are fairly optimized. But this applies to all user defined types, where a move can be arbitrarily expansive or cheap. It's something worth keeping in mind.

T.C. · Accepted Answer · 2018-12-20 11:33:06Z

5

Worth noting that in C++14 and later you can just use a default member initializer:

struct C {
    std::vector<int> a;
    std::string b;
    bool c = false;
};

Also, aggregate initialization generates no constructors. It bypasses them entirely.

answered Dec 20, 2018 at 11:33

T.C.

139k18 gold badges306 silver badges440 bronze badges

Comments

lubgr · Accepted Answer · 2018-12-20 08:06:34Z

3

You can pass an instance of a std::initializer_list<int> like this:

#include <initializer_list>

struct C {
    /* Data members... */

    C(std::initializer_list<int> vecData, std::string str, bool flag) :
        a{vecData}, b{std::move(str)}, c{flag} {}
};

answered Dec 20, 2018 at 8:06

lubgr

38.6k3 gold badges71 silver badges119 bronze badges

Collectives™ on Stack Overflow

How to write proper std::initializer_list constructor

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related