Default constructor missing for a nested class in the copy constructor of a parent class

Question

I have a nested class structure, Class A is parent and Class B is nested under. When I compile the code, the copy constructor of Class A reports that there is no default constructor for the class B.

error: no default constructor exists for class "A::B"

class A{
   -------
   struct B{
     B(var1, var2){}
   };
   B b;
};

A::A(){ b = new B(Var1, Var2) } // default constructor
A::A(a){ } // copy constructor

Any ideas on how to fix this ?

Where do you have an instance of `B`? Does only the copy constructor get the error? — Fred Larson, Apr 04 '14 at 20:42
Show us code that we can actually use and compile to get the same behavior you are experiencing. — David G, Apr 04 '14 at 20:43
He gives you suitable code. Since he doesn't specify it, both of the constructors he shows for A will try to use a default constructor for B. — UpAndAdam, Apr 04 '14 at 20:49
possible duplicate of [How do you use the non-default constructor for a member?](http://stackoverflow.com/questions/2088944/how-do-you-use-the-non-default-constructor-for-a-member) — Fred Larson, Apr 04 '14 at 20:53
@UpAndAdam I have a delete in the destructor which I haven't mentioned in the post. Thats a good catch. — user1801733, Apr 04 '14 at 21:04
Wait, you mean `b` is actually a pointer? You really need to post actual code. — Fred Larson, Apr 04 '14 at 21:11

score 3 · Answer 1 · answered Apr 04 '14 at 20:51

3

You need to use a member initializer in both constructors:

A::A() : b(Var1, Var2) {}
A::A(const A& a) : b(Var1, Var2) {}

answered Apr 04 '14 at 20:51

Fred Larson

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UpAndAdam · Answer 2 · 2014-04-04T20:54:46.267

You defined a constructor taking two arguments B(var1, var2) as such a default constructor is not automatically provided for you.

So you have a couple of options.

Choice 1

Add a default constructor for b in your struct B definition

struct B{
    ....
    B() {};
}

also your syntax below is wrong it should be:

A::A() : b() {};
A::A( const A& a) : b() {};

Choice 2

You could use the non default constructor of B but you have to come up with values from somewhere

A::A() : B( valA, valB ) {};
A::A( const A& a) : b(valA, valB) {};

Choice 3

You probably don't want that and instead should create a copy constructor for B and do this

A::A( const A& a) : b(a.b) {};

`A` does not derive from `B`, but `A` does have a member named `b` that is of type `B`, so the initialization list in the `A` constructor needs to use `b()` instead of `B()`. — Remy Lebeau, Apr 04 '14 at 20:53

Default constructor missing for a nested class in the copy constructor of a parent class

2 Answers2

Choice 1

Choice 2

Choice 3