initializer_list in initialization list

Question

I want initialize base class with std::initializer_list.

struct A : public std::array<int, 4>
{
  // This constructor works fine
  A()
  : std::array<int, 4>{{ 1, 2, 3, 4 }}
  {
  }

  // THIS CONSTRUCTOR FAILS TO COMPILE
  A(std::initializer_list<int> il)
  : std::array<int, 4>{il}
  {
  }
};

GCC error for second constructor is

error: array must be initialized with a brace-enclosed initializer

What I want is to initialize new A instance with initializer_list like this

A var{{ 1, 2, 3, 4 }}

and pass it to base class.

Chnossos · Accepted Answer · 2014-04-13 02:01:18Z

2

The class std::array has no constructor taking a std::initializer_list.

The only way you have is to do it like this :

#include <array>
#include <initializer_list>

struct A : public std::array<int, 4>
{
    A()
    : std::array<int, 4>{{ 1, 2, 3, 4 }}
    {
    }

    A(std::array<int, 4> il)
    : std::array<int, 4>(il)
    {
    }
};

int main ()
{
    A a ({{ 1, 2, 3, 4 }});
}

answered Apr 13, 2014 at 2:01

Chnossos

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9 Comments

user3528030 Over a year ago

seems like std::initializer_list is implicitly casted to std::array here. Would this have a runtime performance impact?

Chnossos Over a year ago

It is not a std::initializer_list that is used here : std::array uses aggregate_initialization. Quoting cppreference : [std::array] combines the performance and accessibility of a C-style array with the benefits of a standard container [...]

ooga Over a year ago

@Chnossos, Although an array can be initialized with list initialization, it cannot be initialized with an actual initializer_list object. Try: auto il = {1,2,3}; array<int,3> a(il);

user3528030 Over a year ago

So should we use aggregate_initialization for fixed size things and initializer_list for variable sized things then?

ooga Over a year ago

@ShitImplementor You can only use an initializer_list object if there's a ctor that accepts one. As for the performance penalty, with move constructors it shouldn't be a problem.

|

ooga · Accepted Answer · 2014-04-13 01:56:54Z

0

It's saying that an array can be initialized with a "brace-enclosed initializer" as in your first ctor, but not with an initializer_list as in your second. There is no array ctor that takes an initializer_list. In fact, there are no ctors for arrays except for the implicitly-declared special member functions.

answered Apr 13, 2014 at 1:56

ooga

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