C++ - Calling a constructor other than the default when an object is instantiated in another class

Question

Feel free to edit the title I'm not sure how to phrase this.

I'm trying to figure out how to call a class's constructor other than the default when it is instantiated in another class. What I mean is this...

class A
{
public:
    A(){cout << "i get this default constructor when I create a B" << endl;}
    A(int i){cout << "this is the constructor i want when I create a B" << endl;}
};

class B
{
    A a;
};

int main()
{
    B *ptr = new B;
    return 0;
}

I've done some searching but I don't see a way to do what I want. I thought maybe in B's declaration I could do A a(5) but that doesn't work.

Thanks

Just a note: that B *ptr = new B; can be replaced by the safer B obj;. You won't have to free any memory. If you still need a pointer, consider std::unique_ptr. — Qaz
– Qaz, Commented Aug 30, 2012 at 23:14
possible duplicate of c++ calling non-default constructor as member — jogojapan
– jogojapan, Commented Aug 31, 2012 at 0:54

Community · Accepted Answer · 2017-05-23 12:33:58Z

10

You can do that with a constructor initialization list (you might also want to look at this question and others similar to it).

This means that you will have to manually write a constructor for B:

class B
{
    A a;

    public: B() : a(5) {}
};

See it in action.

edited May 23, 2017 at 12:33

CommunityBot

11 silver badge

answered Aug 30, 2012 at 23:13

Jon

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1 Comment

loop Over a year ago

Thanks that worked. I like your example tool it's pretty cool.

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