c++ call constructor from constructor

Question

Hi *,

As an c# developer I'm used to run through constructors:

class Test {
    public Test() {
        DoSomething();
    }

    public Test(int count) : this() {
        DoSomethingWithCount(count);
    }

    public Test(int count, string name) : this(count) {
        DoSomethingWithName(name);
    }
}

Is there a way to do this in c++ ?

I tried calling the Class name and using the 'this' keyword, but both fails.

Answer 1

If I understand your question correctly, you're asking if you can call multiple constructors in C++?

If that's what you're looking for, then no - that is not possible.

You certainly can have multiple constructors, each with unique argument signatures, and then call the one you want when you instantiate a new object.

You can even have one constructor with defaulted arguments on the end.

But you may not have multiple constructors, and then call each of them separately.

Answer 2

No, in C++ you cannot call a constructor from a constructor. What you can do, as warren pointed out, is:

• Overload the constructor, using different signatures
• Use default values on arguments, to make a "simpler" version available

Note that in the first case, you cannot reduce code duplication by calling one constructor from another. You can of course have a separate, private/protected, method that does all the initialization, and let the constructor mainly deal with argument handling.

Answer 3

No, you currently can't call one contructor from another in C++ (delegating constructor, it's called, I think).

This will change in C++ 09, which such syntax (example taken from Wikipedia) :

class SomeType
{
  int number;

public:
  SomeType(int newNumber) : number(newNumber) {}
  SomeType() : SomeType(42) {}
};

Answer 4

Unfortunately there's no way to do this in C++.

two ways of simulating this:

1) You can combine two (or more) constructors via default parameters:

class Foo {
 public:
   Foo(char x, int y=0);  // combines two constructors (char) and (char, int)
   ...
 };

2) Use an init method to share common code

class Foo {
 public:
   Foo(char x);
   Foo(char x, int y);
   ...
 private:
   void init(char x, int y);
 };

 Foo::Foo(char x)
 {
   init(x, int(x) + 7);
   ...
 }

 Foo::Foo(char x, int y)
 {
   init(x, y);
   ...
 }

 void Foo::init(char x, int y)
 {
   ...
 }

see this link for reference

Answer 5

It is worth pointing out that you can call the constructor of a parent class in your constructor, e.g.:

class A{ .... };

class B: public A { B() : A() { ... do more stuff... } };

But, no, you can't call another constructor of the same class.

Answer 6

I believe you can call a ctor from a ctor. It will compile and run. I recently saw someone do this and it ran on windows and linux.

It just doesn't to what you want. The inner ctor will construct a temporary local object which gets deleted once the outer ctor returns. They would have to be different ctors as well or you would create a recursive call.

Ref: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.3