Initialize first entries of an array of objects in class constructor initialization list

Question

If I have class A that declares Class B and instantiates an array of it.

class A{
    class B{
        int x, y;
    };
    B arrB[10];

 public:
    A();
};

Then, my issue is that I would like to initialize the first two objects of "arrB" in Class A using initialization list:

A::A(): arrB[0](1, 2), arrB[1](3, 4) {}

But the compiler does not accept it.

Can I initialize specific objects of the array or not? If yes, how to do it?

Thanks

Is there a reason you need this in the initialization list instead of just in the constructor function body? — scohe001
– scohe001, Commented Jul 11, 2019 at 19:24
Maybe something like: arrB{{1,2},{3,4},{},{},{},{},{},{},{},{}} or arrB{{1,2},{3,4}} however rest of the elements gets random values and it must be a struct or public x,y — KIIV
– KIIV, Commented Jul 11, 2019 at 19:29
@KIIV: The second form is fine; the rest of the elements will be zero initialized — AndyG
– AndyG, Commented Jul 11, 2019 at 19:34
The compiler complains on the second form as it gives error asking about init of the rest of the array items — mnabil
– mnabil, Commented Jul 11, 2019 at 19:38
After the edits in question it won't work. It'll work with struct { int x, y; }; — KIIV
– KIIV, Commented Jul 11, 2019 at 19:45

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

The problem is that B hides its members by default as private because it is a class. Declare B a struct, or expose int x, y as public to be able to use aggregate initialization:

class A{
    class B{
        public:
        int x, y;
    };
    B arrB[10] = {{1,2}};

 public:
    A();
};

The second problem is that you're not using aggregate initialization properly.

A::A(): arrB[0](1, 2), arrB[1](3, 4) {}

Should be

A::A(): arrB{{1, 2}, {3, 4}} {}

Demo

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 11, 2019 at 19:29

AndyG

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Comments

NathanOliver · Accepted Answer · 2019-07-11 19:43:05Z

In order to initialize an array in the class member initialization list you need to use curly braces like you would if you were initializing an array in a function.

Thus if you wanted to initialize the first and second element of the array you would need to use at least

A() : arrB{{1,2}, {3,4}} {}

as the first set ({1,2}) says make arrB[0] a B with x and y initialized to 1 and 2 respectively. The second set ({3,4}) says make arrB[1] a B with x and y initialized to 3 and 4 respectively.

You do have to do one thing though in order to make this work. You either need to make B and aggregate by making x and y public, or you can provide a constructor for B that takes to values. Doing that lets you have either

class A{
    class B{
    public:
        int x, y;
    };
    B arrB[10];

 public:
    A() : arrB{{}, {3,4}} {}
};

int main() {
    A a;
}

or

class A{
    class B{
        int x, y;
    public:
        B() : x(), y() {} // need this so default instances are value initialized (zeroed in this case)
        B(int x, int y) : x(x), y(y) {}
    };
    B arrB[10];

 public:
    A() : arrB{{}, {3,4}} {}
};

int main() {
    A a;
}

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