call constructor of class with brace enclosed initilizer list

Question

How do I need to define a constructor such that i can construct a class A like A({a,b,c}). This arised in an urge to avoid using a variadic constructor which seems dangerous.

Code:

#include <vector>

class A{
public:
A(const unsigned int a, const unsigned int b, const unsigned int c) /*: A({a,b,c})*/ {} 
A(const std::vector<unsigned int> v) {}
};


int main(){
A(1,2,3); //works
A({1,2,3}); // doesnt compile

return 0;
}

My aim is to construct a network with arbitrarily many hidden layers. The vector is supposed to contain the amount of nodes contained by the corresponding layer.

edit:

/home/simbla/qt_projects/minimalWorkingExample/main.cpp:158: error: call of overloaded 'A(<brace-enclosed initializer list>)' is ambiguous
 A({1,2,3});
          ^

Solution:

use std::initializer_list instead of std::vector

You should look at: http://en.cppreference.com/w/cpp/utility/initializer_list — Amadeus, Mar 13 '18 at 16:32

StoryTeller - Unslander Monica · Accepted Answer · 2018-03-14T08:41:43.430

3

There's actually more constructors in your question than you let on. Here's two more, courtesy of your compiler:

A(A const&) = default;
A(A &&) = default;

Now the interesting bit is what happens when you write another initializer, A{1, 2, 3}. That calls your first user defined c'tor. Nothing novel there. But when you write A({1, 2, 3}), overload resolution has two options. Either construct a temporary A{1, 2, 3} and copy it. Or construct a vector for the other user provided c'tor.

In both cases it's a user defined conversion sequence. Neither is better than the other, so you get a nice flashing error due to ambiguity.

The solution? Either use brace initialization A{{1,2,3}}; or supply an initializer_list constructor, to better match the initializer list argument.

edited Mar 14 '18 at 08:41

answered Mar 13 '18 at 16:51

StoryTeller - Unslander Monica

165,132
21
377
458

@KillzoneKid - Interesting. I checked with Clang, [and it did build](http://coliru.stacked-crooked.com/a/4fc05c3a28918b04). GCC seems to disagree. I wonder... – StoryTeller - Unslander Monica Mar 13 '18 at 17:02
VC++ doesn't like it too – Killzone Kid Mar 13 '18 at 17:03
1

@KillzoneKid - VC is rarely an indication of anything. I need to check this more thoroughly. – StoryTeller - Unslander Monica Mar 13 '18 at 17:03
1

@KillzoneKid - I've drawn a blank. So [here's a followup question](https://stackoverflow.com/questions/49264846/can-the-explicitness-of-a-constructor-supposedly-be-ignored-due-to-the-form-of-i) if you wish to follow this saga as well. – StoryTeller - Unslander Monica Mar 13 '18 at 19:57
How is `A({1, 2, 3})` thought by the compiler to possibly mean `A(A{1, 2, 3})`? In other words, why does overload resolution have two options? – Swiss Frank Oct 05 '20 at 16:38
1

@SwissFrank - The compiler sees you passing `{1, 2, 3}` as argument to the cast expression `A(...)`. Now it has to match it to a parameter of one of `A`'s constructors. Since we can initialize `A{1, 2, 3}`, it's obviously a valid initializer for an `A`. So, it becomes `A({1, 2, 3} -> A)` or `A({1, 2, 3} -> vector<>)`. – StoryTeller - Unslander Monica Oct 05 '20 at 17:05

score 1 · Answer 2 · answered Mar 13 '18 at 16:29

1

A(const std::vector<unsigned int> v) {}

No, vector is not the right thing. You want

A(const std::initializer_list<unsigned int> v) {}

answered Mar 13 '18 at 16:29

iBug

35,554
7
89
134

thank you. i think i even read that today. Couldn't find it again tho. – Gladaed Mar 13 '18 at 16:30

score 1 · Answer 3 · answered Mar 13 '18 at 16:33

I think you are looking for std::initializer_list:

struct S {
    std::vector<int> v;
    S(std::initializer_list<int> l) : v(l) {
        std::cout << "constructed with a " << l.size() << "-element list\n";
    }
};

int main() {
    S s({1, 2, 3, 4, 5});

}

call constructor of class with brace enclosed initilizer list

3 Answers3

Linked