Dynamic Array allocation of a type with a custom constructor (C++) [duplicate]

Question

I have the following class and struct (stripped down for simplicity)

a.h

class A {
    B *myArray;
}

a.cc

A::A() {
    myArray = new B[1];
}

b.h

struct B {
    int number;
}

b.cc

B::B(int n): number(n)  {
}

As you can see the class B only has the constructor that takes an integer, so when I do myArray = new B[1] it throws no matching constructor for initialization of 'B[1]'.

How can I fix this? I've tried doing an array of pointers instead of an array of type B but it doesn't work because of the way information is entered into the program.

I cannot add a new constructor to B. I can only add private variables and methods. I can only use the following libraries: iostream, fstream, sstream, iomanip, string, and utility.

Use a std::vector instead. It has functionality built in to handle non-default constructable types. For every dynamic array, there is a vector that will solve it's problems — NathanOliver
– NathanOliver, Commented Oct 12, 2018 at 18:14
@MarkDodds All of the headers you name are in the same library as <vector>. That's the standard library. It wouldn't be surprising for one of the ones you named (like string) to be using <vector>. — François Andrieux
– François Andrieux, Commented Oct 12, 2018 at 18:24

scohe001 · Accepted Answer · 2018-10-12 18:26:44Z

2

Since you define a constructor for B, you no longer have a default constructor defined for you.

When you do myArray = new B[1]; you're trying to build an array of size 1 while calling B's default constructor, but no such constructor exists!

This can be fixed by either making your own default B constructor, or calling the constructor you made when allocating. The latter can be done with:

myArray = new B[1]{5};

See it work here: ideone

edited Oct 12, 2018 at 18:26

answered Oct 12, 2018 at 18:18

scohe001

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4 Comments

Jesper Juhl Over a year ago

"you no longer have a default constructor defined for you" - can be fixed by writing one, or if the default one is sufficient, declare it as = default;.

NathanOliver Over a year ago

@JesperJuhl Which the OP cannot do

scohe001 Over a year ago

@JesperJuhl good point. Added that creating your own default c'tor is definitely an option.

user4581301 Over a year ago

Pity you can't just take advantage of aggregation.

Matthieu Brucher · Accepted Answer · 2018-10-12 18:15:28Z

1

The issue is that you don't have a default constructor to B. You have to provide one or at least a default value for the only parameter (also the constructor should be tagged as explicit.

Another option is to use a vector. You should avoid any explicit delete anyway, so it's a better option to what you have.

answered Oct 12, 2018 at 18:15

Matthieu Brucher

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