Reusable constructors C++

Question

One of the corner stones of OOP is reusing code instead of repeat it over and over. Thus, your projects shorten and get more readable.

C++ gives you all the tools you need to reuse methods instead of repeating the code. Although when it comes to constructors I do not know how to reuse them.

I am not talking of heritage or how to send a message to the father. I am talking about reusing the constructor of the class itself.

The analogy in JAVA is something like this:

public Foo() {
    this(0,0,0);//Not needed in this case, just to clarify
}

public Foo(Foo f){
    this(f.getA(), f.getB(), f.getC());
}

public Foo(int a, int b, int c) {
    this.a = a;
    this.b = b;
    this.c = c;
}

My question is, is there any syntaxis in C++ that allows you to do so?

Answer 1

C++11 has added constructor delegation and constructor inheritance.

To inherit constructors, a using-declaration is required:

class Base { ... };

class Derived : public Base
{
    using Base::Base;
};

To delegate, use the ctor-initializer, but specify another constructor in the same class, instead of any subobjects (all base and member subobjects will be initialized by the constructor delegated to):

class Another : public Base
{
    int member;
    Another(int x)
        : Base(), member(x) // non-delegating constructor initializes sub-objects
    {}


    Another(void)
        : Another(5) // delegates -- other constructor takes care of Base and member
    {}
};

And perfect forwarding can also come in handy.

Answer 2

Others already answered about C++11, but for C++03 there's a possible workaround: using a base class with needed constructor(s).

struct foo_base {
    foo_base(int a, int b, int c) : a(a), b(b), c(c) { }
    int a, b, c;
};

struct foo : foo_base {
    foo() : foo_base(0, 0, 0) { }
    foo(const foo& other) : foo_base(other.a, other.b, other.c) { }
    foo(int a, int b, int c) : foo_base(a, b, c) { }
};

Of course, you need to consider whether it's worth the boilerplate for your purposes.

Answer 3

The generally accepted soultion for current compilers is to do this:

class Bar{
pubilc:    
Foo() {
   init(0,0,0);
}

Foo(const Foo &f){
  init(f.getA(), f.getB(), f.getC());
}

Foo(int a, int b, int c) {
  init(a,b,c);
}

private:

void init(int a, int b, int c){
  this->a = a;
  this->b = b;
  this->c = c;
}
};

While this may seem like over kill in this example, that is only because of the simplicity of the example. In a real world application this would actually bring benefits in terms of reduction of repeated code.

Answer 4

OK C++11 covers what you need.

But your simple case has an easy solution:

/* This one is covered by providing default parameters see below.
public Foo() {
    this(0,0,0);//Not needed in this case, just to clarify
}

 This is done automatically by the compiler.
 You do not need to write any code for this:
public Foo(Foo f){
    this(f.getA(), f.getB(), f.getC());
}
The compiler generated version actually looks like this:
public Foo(Foo const& f)
    : a(f.a)
    , b(f.b)
    , c(f.c)
{}



*/

// Now you can use all three methods and they work fine:
public Foo(int a = 0, int b = 0, int c = 0)
    : a(a)
    , b(b)
    , c(c)
{}

F   f1;        // default construct no parameters: uses the three parameter version
F   f2(f1);    // Copy constructed. Generated by the compiler.
F   f3(1,2,3); // Nomal constructor