Class with std::array of objects without default constructors

Question

So let's assume I have the following class

class NoDefaultConstructor {
    NoDefaultConstructor() = delete;
    ...
};

And I have another class which has an array of type NoDefaultConstructor and other members

class Wrapper {
    std::array<NoDefaultConstructor, 2> arr;
    ... 
};

How can I initialize the array in the constructor for Wrapper (maybe in the initializer list using an std::intializer_list)?

More specifically, is the only way I can pass on arguments to the array constructor in the initializer list for Wrapper to have a construct similar to the following? I was thinking of doing this because the size of the array might change in the future.

template <typename... Values>
Wrapper(Values&&... values) : arr{std::forward<Values>(values)...} {}

You could store std::unique_ptr<NoDefaultConstructor> instances in the array instead of just NoDefaultConstructor. — sjrowlinson
– sjrowlinson, Commented Jul 7, 2016 at 20:53
I explored that idea but I didn't want to use it because I could rather have used an std::vector instead in that case — Curious
– Curious, Commented Jul 7, 2016 at 20:53
Search for questions on how to initialize data members w/o default constructors. There are plenty of those on SO. — juanchopanza
– juanchopanza, Commented Jul 7, 2016 at 20:55
@juanchopanza this is slightly different from the general case — Curious
– Curious, Commented Jul 7, 2016 at 21:01

Curious · Accepted Answer · 2016-07-07 23:03:51Z

8

std::array is required to be an aggregate. Therefore it has no nontrivial constructors, but can be initialized using aggregate initialization. Note that aggregate initialization involves a braced-init-list (that is, a brace-enclosed list of initializers) but not an std::initializer_list object.

class Wrapper {
  public:
    Wrapper() : arr {MakeNoDefaultConstructor(123),
                     MakeNoDefaultConstructor(456)} {}
    //              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ braced-init-list
  private:
    std::array<NoDefaultConstructor, 2> arr;
};

EDIT A constructor with variadic arguments could be possible here like so

#include <array>
struct NoDefault {
    NoDefault() = delete;
    NoDefault(int) {}
};
struct Wrapper {
    template <typename... Args>
    Wrapper(int b_in, Args&&... args) : b{b_in}, a{args...} {}
    int b;
    std::array<NoDefault, 3> a;
};
int main() {
    std::array<NoDefault, 2> a {12, 34};
    Wrapper w {23, 12, 34, 19};
}

This of course can be further tightly constrained by adding enable_ifs

edited Jul 7, 2016 at 23:03

Curious

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answered Jul 7, 2016 at 20:56

Brian Bi

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6 Comments

Curious Over a year ago

Is there no way to forward some arguments to this constructor? Is the only way to use variadic templates?

Brian Bi Over a year ago

@Curious I don't understand, could you perhaps elaborate?

Curious Over a year ago

I wanted the class constructor to accept parameters that would then get forwarded to the array constructor

Taywee Over a year ago

I don't see the variadic templates.

Brian Bi Over a year ago

@Curious To what array constructor? Like I said, std::array has no nontrivial constructors. Are you saying you want Wrapper to exhibit the same construction semantics as the std::array type it wraps? If that's the case, you should make Wrapper into an aggregate as well. coliru.stacked-crooked.com/a/d6affc3571b9c956

|

Jonathan Wakely · Accepted Answer · 2016-07-07 21:17:16Z

3

How can I initialize the array in the constructor for Wrapper (maybe in the initializer list using an std::intializer_list)?

Just as you would for any other member, but use uniform initialization because std::array doesn't have any constructors.

More specifically, is the only way I can pass on arguments to the array constructor in the initializer list for Wrapper to have a construct similar to the following?

No, why would you need to use a variadic templates for a fixed number of arguments?

Just ... write the constructor, with arguments:

class Wrapper {
    std::array<NoDefaultConstructor, 2> arr;

    Wrapper(const NoDefaultConstructor& a, const NoDefaultConstructor& b)
    : arr{ a, b }
    { }
};

answered Jul 7, 2016 at 21:17

Jonathan Wakely

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1 Comment

Curious Over a year ago

Edited my question regarding why I was thinking of using variadic arguments, does it make more sense now?

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