How to define a vector class using std::array with aggregate initialization?

Question

I define a class like this

#include<array>
template<class T,size_t D> 
class vec{
    private:
        std::array<T,D> arr;
    public:
        vec(std::initializer_list<T> l) : arr(l){ }
};

Then I construct the object of this class like this.

vec<int,3> v = {1,2,3};

Then I get an error

main.cpp:5:26: error: cannot bind non-const lvalue reference of type ‘std::initializer_list<int>&’ to an rvalue of type ‘std::initializer_list<int>’
     vec<int,3> v = {1,2,3};
                          ^
In file included from main.cpp:1:
vec.hpp:12:9: note:   initializing argument 1 of ‘vec<T, D>::vec(std::initializer_list<_Tp>&) [with T = int; long unsigned int D = 3]’
         vec(std::initializer_list<T>& l) : arr(l){ }

I wonder how can I make the aggregated initialization in my constructor valid.

NathanOliver · Accepted Answer · 2020-01-14 15:11:10Z

2

A std::array is not constructable from a std::initializer_list. You have two options to fix this if you want to keep the std::array member. You can make vec and aggregate by making arr public and removing the constructor like

#include<array>
template<class T,size_t D> 
class vec{
    public:
        std::array<T,D> arr;
};

int main()
{
    vec<int,3> v = {1,2,3};
}

or you keep it as you have it but instead you iterate over arr and assign it values from the initializer list. That would look like

#include<array>
template<class T,size_t D> 
class vec{
    private:
        std::array<T,D> arr;
    public:
        vec(std::initializer_list<T> l)
        {
            std::copy_n(l.begin(), std::min(l.size(), arr.size()), arr.begin());
        }
};

int main()
{
    vec<int,3> v = {1,2,3};
}

This method does require the array members to be default constructable. If you can't/don't want to guarantee that then you need option one or switch to using a different type for arr like std::vector.

edited Jan 14, 2020 at 15:11

answered Jan 14, 2020 at 15:02

NathanOliver

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3 Comments

Richard Over a year ago

Thank you very much. It really helps. Do you mind explaining more to me about the default constructor? I wonder if I have two std:: array members arr1 and arr2 in class vec, will v={1,2,3} also work? Thank you.

NathanOliver Over a year ago

@Richard In my second example since we are in the body of the constructor that means that arr was already default constructed. In order for that to happen that means T needs to be a type that has a default constructor. If T is a type that doesn't have a default constructor then you'll get an error because that means it can't default construct arr. You can you either method for two arrays. You would just need to use two lists like vec v = {{1,2,3},{3,4,5}}

Richard Over a year ago

Yeah, I understand.Thank you very much for your help.

Ladence · Accepted Answer · 2020-01-14 14:59:21Z

0

std::array does not have any constructors. It has only aggregate initializer. If you replace std::array by std::vector it should works because vector has initializer list constructor.

answered Jan 14, 2020 at 14:59

Ladence

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