c++ Using overloaded constructor on object composition

Question

may I know how do i use a overloaded constructor for object composition in another class, here's an example code:

class A {
    int a;

    A( int inputA ) { a = inputA; }
};

class B {
    A objectA;

    B( A inputObjectA ) { objectA = inputObjectA; }
};

The error with the compiler was there is no default constructor A::A() ?

Is there any way to edit the code in the parameter of B's constructor to accept the overloaded constructor in A?

Use constructor initializer list. – Ivan Aksamentov - Drop Nov 10 '13 at 05:56 — Ivan Aksamentov - Drop, Nov 10 '13 at 05:56

Basilevs · Accepted Answer · 2013-11-10T06:22:22.547

2

If a child has no default constructor, parent's constructor should pass parameters.

class A {
public:
    int a;
    A( int inputA ) { a = inputA; }
};

class B {
    A objectA;
public:
    B( A inputObjectA ): objectA(inputObjectA.a) { }
};

http://ideone.com/QavhfY

In your case it would be appropriate to use a copy constructor for A, instead of making its member public:

class A {
    int a;
public:
    A( int inputA ) { a = inputA; }
};

class B {
    A objectA;
public:
    B(A inputObjectA ): objectA(inputObjectA) { }
};

http://ideone.com/wX1clr

edited Nov 10 '13 at 06:22

answered Nov 10 '13 at 05:36

Basilevs

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hi, for your solution to work, i MUST have a default constructor in class A correct?, so no matter what i will need a default constructor? If not, `B( A inputObjectA ): objectA(inputObjectA) { }` will trigger the default constructor not found – curiosity Nov 10 '13 at 05:45
There is no default constructor for A in my solutions. And no, there won't be compiler errors. I've added links to ideone as a proof that code compiles fine. – Basilevs Nov 10 '13 at 05:47
thanks for your explanation, however my case study will need B to accept a Class A object without referencing. – curiosity Nov 10 '13 at 05:56
I've removed references. They just help to avoid extra copies. – Basilevs Nov 10 '13 at 05:58
Oops, A(A that) constructor is rejected by C++11 :( – Basilevs Nov 10 '13 at 06:02
I've removed it for A to rely on a default copy constructor. – Basilevs Nov 10 '13 at 06:09

Adam namrog84 · Answer 2 · 2013-11-10T05:36:40.213

In your class B object you have

A objectA;

That is when the default constructor is being called, so in there you need to either have a designated default constructor, or you can alternatively use a default parameter, such as

A( int inputA = 0 ) { a = inputA; }

This will then take the role that set's the integer a to 0, when you call the default constructor, it really would achieve the same as

A() { a = 0; }

though you don't have to set a if you don't want to, but since you declared a non default constructor, it no longer implicitly creates one for you. So you will need at minimum of

A(){ }

Another way you could do it is

class A {
    int a;
    void setSomething(int val) { a = val; }
};

class B {
    A objectA;
    B( A inputObjectA ) { objectA = inputObjectA; }
};

This works because you never declared any constructor, so it will implicitly create a default one for you

Lastly, this is one other way you could do it, probably closest to your original code with only 3 characters added, should fix everything:

class A {
    int a;

    A( int inputA ) { a = inputA; }
};

class B {
    A objectA(0);

    B( A inputObjectA ) { objectA = inputObjectA; }
};

is there any way to edit the code in the parameter of B's constructor to accept the overloaded constructor in A. — curiosity, Nov 10 '13 at 05:33
Also when you define a constructor for a class the default constructor does not get called anymore — Vahid Nateghi, Nov 10 '13 at 05:33
The error is not from there, it's from `A ObjectA` so this line should change — Vahid Nateghi, Nov 10 '13 at 05:38

score 0 · Answer 3 · edited May 23 '17 at 11:57

Just pass A()'s parameters to constructor of B(). This solution works for A which is neither default constructable, nor copy or move constructable/assignable.

class A 
{
    int a;

    // Just to demonstrate non copy-move-ness (can be removed)
    A() = delete;
    A( A& ) = delete;
    A( A&& ) = delete;
    A& operator=( A& ) = delete;
    A& operator=( A&& ) = delete;
public:
    A( int inputA ): a(inputA ) {}
};

class B 
{
    A objectA;
public:
    B( int inputA ): objectA(inputA ) { }
};


int main() 
{
    B(42);
}

Proof

Tip: don't use assignment in constructor body, use constructor init lists instead.

All this info you could get from any modern C++ book.

score -1 · Answer 4 · answered Nov 10 '13 at 05:40

-1

You need a default constructor, because you have an instance of class A inside class B. If you read a bit about the order in which constructors are called, you'll notice that first there are called default constructors for members, and then the main class constructor. So - in your example - first the compiler tries to init objectA with a default value, and it fails, because there is no default constructor.

answered Nov 10 '13 at 05:40

Paweł Stawarz

3,952
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There is a way to pass arguments to nondefault constructors for such cases. B constructor should be modified to do so. See my answer. – Basilevs Nov 10 '13 at 05:54
Wrong. There is no absolute need in default constructor even if member is by value. – Ivan Aksamentov - Drop Nov 10 '13 at 05:58
While what you both say is true, the content of the question was different while I was providing the answer for it. The user didn't ask "how to change the bode", but "do I need a default constructor". So while changing the code to use an initializer list is a good idea, the answer to the primal question is "yes, you need it because...". – Paweł Stawarz Nov 10 '13 at 15:25

c++ Using overloaded constructor on object composition

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