Building constructors with shared functionality

[Aguilar, James]

I want to call a constructor from another constructor without constructing
an object. In other words, I want to partially initialize the object I'm
working with with one constructor, and finish the job with another.

class Foo
{
public:
Foo();
Foo(int i);
private:
int a, b;
};

Foo::Foo()
{
a = b = 0;
}

Foo::Foo(int i)
{
Foo(); //or should it be Foo::Foo()?
a = i;
}

Is there any way to do this in C++ that does not involve private helper
methods?

 

[John Harrison]

I want to call a constructor from another constructor without
constructing
an object. In other words, I want to partially initialize the object I'm
working with with one constructor, and finish the job with another.

class Foo
{
public:
Foo();
Foo(int i);
private:
int a, b;
};

Foo::Foo()
{
a = b = 0;
}

Foo::Foo(int i)
{
Foo(); //or should it be Foo::Foo()?
a = i;
}

Is there any way to do this in C++ that does not involve private helper
methods?

No. But if you really need a partially initialised object that might be a
clue that what you really need is to split your object into a base class
and a derived class. While the derived class is being created the base
class will be created first and that is your 'partially initialised
object'.

john

 

[Sharad Kala]

I want to call a constructor from another constructor without constructing
an object. In other words, I want to partially initialize the object I'm
working with with one constructor, and finish the job with another.

class Foo
{
public:
Foo();
Foo(int i);
private:
int a, b;
};

Foo::Foo()
{
a = b = 0;
}

Foo::Foo(int i)
{
Foo(); //or should it be Foo::Foo()?
a = i;
}

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.3
You can't call a constructor. Foo() in your above definition of Foo(int i)
is a temporary object, it doesn't call the constructor. Instead write some
private member function init and call it.
-Sharad

 

[Alf P. Steinbach]

* Sharad Kala:

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.3
You can't call a constructor

.... on the 'this' object (without using placement new, which you
shouldn't anyway).

Read the FAQ and HS carefully.

. Foo() in your above definition of Foo(int i)
is a temporary object, it doesn't call the constructor. Instead write some
private member function init and call it.

Right.

 

[Sharad Kala]

* Sharad Kala:

... on the 'this' object (without using placement new, which you
shouldn't anyway).

Read the FAQ and HS carefully.

Well, I have I shouldn't have been in a haste though.