Problem with object declaration/initialization as a private member on a different class

Question

Sorry if this has been asked before, I can't seem to find anything. I'm not sure how to search for this.

I have something like this:

class A {
    private:
        int x;
        int y;
    public:
        A(int, int);
}

class B {
    private:
        A a(3, 4); // Doesn't compile because of this line
    public:
        B();
}

The only way I could think to solve this was making a a pointer to A and then do a = new A(3, 4); inside B's constructor. But I don't want a to be a pointer.

What's the correct way to solve this?

Are all your defined types simple POD-like integral types? If so you could create them as non-type template parameters. eg. A<3, 4> a;. There are constrains to this approach though, one being those init values must be known at compile-time. — greatwolf
– greatwolf, Commented Mar 14, 2011 at 1:41

Jonathan Grynspan · Accepted Answer · 2011-03-14 00:50:48Z

2

You tag B's constructor with a "member initialization list". Instead of:

B::B() {
    ...
}

You do this:

B::B() : a(3, 4) {
    ...
}

Or if the constructor is defined in the header:

class B {
    private:
        A a;
    public:
        B() : a(3, 4) {
            ...
        }
};

answered Mar 14, 2011 at 0:50

Jonathan Grynspan

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6 Comments

rfgamaral Over a year ago

Isn't there any other way? What if I had dozens of variables to initialize? The code would look really weird.

Steve Jessop Over a year ago

@Nazgulled: C++ does look weird, if you aren't familiar with it. It's not actually a problem until you have dozens of data members and more than one constructor.

Jonathan Grynspan Over a year ago

@Nazgulled: C++ looks weird. That's part of the language. Get over it. That said, if you really hate the syntax, and class A has a default constructor, it will be used automatically; you can then assign a new value to a (a = A(3, 4);) in the constructor, but that's inefficient. Better to use the initialization list where possible as it can be better-optimized and produce smaller/faster code.

rfgamaral Over a year ago

Can I assume that that would be the same as declaring a setVals method to be used in B's constructor as a.setVals(3,4);? Which is also inefficient?

Jonathan Grynspan Over a year ago

That plus the cost of actually replacing the contents of a with the contents of the temporary instance of A. If you haven't defined an assignment operator for A, then that's essentially a call to memcpy()--if you have, it's the cost of that method. The total cost of doing a = A(3, 4) will thus vary based on the actual type of class A. Just get over your distaste of the syntax and use the initialization list. It will do exactly what you need and is the proper way to initialize fields in C++.

|

Community · Accepted Answer · 2017-05-23 10:33:09Z

2

class B {
    private:
        A a;
    public:
        B() : a(3,4) {}
};

In a wider sense, the solution is to learn C++ by reading a book about it. Yes, that's snarky, but the point of tutorials is that they introduce concepts in a sensible order, and when they tell you about data members they will simultaneously tell you how to initialize them.

edited May 23, 2017 at 10:33

CommunityBot

11 silver badge

answered Mar 14, 2011 at 0:50

Steve Jessop

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Comments

dfan · Accepted Answer · 2011-03-14 00:50:22Z

0

If what you want is for B.a to be initialized with the arguments 3 and 4, then you do that in B's constructor, e.g.,

class B {
    private:
        A a;
    public:
        B(): a(3, 4) {}
}

answered Mar 14, 2011 at 0:50

dfan

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