How to instantiate derived classes without having to rewrite code?

Question

Let's say I have a:

class A {
    A(int i);
};

class B : A {
};

I cannot instanciate B(3) for instance, as this constructor is not defined. Is there a way to instanciate a B object that would use the A constructor, without having to add "trivial" code in all derived classes? thanks

thanks

score 6 · Accepted Answer · answered Nov 05 '11 at 19:43

6

C++11 has a way:

class A {
public:
    A(int i);
};

class B : A {
public:
    using A::A; // use A's constructors
};

answered Nov 05 '11 at 19:43

Pubby

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For completeness: and it's impossible in C++03. – Cat Plus Plus Nov 05 '11 at 19:45
1

Weehoo. I didn't realize constructor forwarding enabled that. +1 This is sweet for template meta-programming too – sehe Nov 05 '11 at 19:47
@Pubby8: that must be the reason I hadn't found out yet! – sehe Nov 05 '11 at 19:50

score 2 · Answer 2 · edited May 23 '17 at 11:55

If you're using C++03 this is the best thing I can think of in your situation:

class A {
public:
    A(int x) { ... }
};

class B : public A {
public:
    B(int x) : A(x) { ... }
}

You may also want to check out the link below, which is a C# question but contains a more detailed answer regarding why constructors can act this way:

C# - Making all derived classes call the base class constructor

score 2 · Answer 3 · answered Nov 05 '11 at 23:40

2

as user491704 said it should be something like this

class mother {
public:
 mother (int a)
 {}
 };

class son : public mother {
public:
 son (int a) : mother (a)
 { }
   };

Here is a link for the Tutorial

answered Nov 05 '11 at 23:40

andrewmag

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How to instantiate derived classes without having to rewrite code?

3 Answers3