Retaining *this in method chaining

[Ashwin Nanjappa]

Hi folks,

I've got a class setup as shown below:

class A
{
public:
A& call()
{ return *this; }

void init()
{ }
};

class B : public A
{
public:
void init()
{ }
};

B b;
b.call().init();

This calls class A's init instead of that of B. This is because call()
returned object of type A instead of B. Is there any other way to
design these classes so that functions like call() know how to return
their actual object instead of the base object?

Why do I want this crazy setup? I want to retain the ability to do
method chaining (b.call1().call2().call3() ...) But, doing it on
inherited classes complicates matters like this :-(

Rgds,
~ash

 

[Ian Collins]

Hi folks,

I've got a class setup as shown below:

class A
{
public:
A& call()

virtual A& call()

{ return *this; }

void init()
{ }
};

class B : public A
{
public:

B& call()
{ return *this; }

 

[Rolf Magnus]

Hi folks,

I've got a class setup as shown below:

class A
{
public:
A& call()
{ return *this; }

void init()
{ }
};

class B : public A
{
public:
void init()
{ }
};

B b;
b.call().init();

This calls class A's init instead of that of B.

B doesn't even have its own implementation.

This is because call() returned object of type A instead of B. Is there
any other way to design these classes so that functions like call() know
how to return their actual object instead of the base object?

Yes. Make A::call() virtual. If your text book doesn't describe how to use
polymorphism in C++, throw it away, because that's a basic language
feature.

 

[Ashwin Nanjappa]

Yes. Make A::call() virtual. If your text book doesn't describe how to use
polymorphism in C++, throw it away, because that's a basic language
feature.

Thanks, this is exactly what I was looking for.

Regards,
~ash

 

[Ashwin Nanjappa]

virtual A& call()




B& call()
{ return *this; }

Thanks Ian. In my case, making init() virtual did the trick.

Regards,
~ash

 

[Rossonera]

Hi,
I think there are two ways to resolve your problem.
1. add B& call() {} member function.
2. declare void init() as virtual in class A.
both of them can resolve your problem.

 

[Ashwin Nanjappa]

Hi,
I think there are two ways to resolve your problem.
1. add B& call() {} member function.
2. declare void init() as virtual in class A.
both of them can resolve your problem.

Yes, this is why I was a bit confused. Solution 2 works for me. This
is because this particular class(es) have a lot of methods that can be
chained but only one init call.

~ash