shared_ptr + this

[ralpe]

Hi,

I have a question regarding boost::shared_ptr. I want to let
shared_ptr take care of my objects' lifetimes, so I change
all my function signatures so that they accept shared_ptrs
instead of raw pointers.

I wonder how to deal with cases where a this pointer is
passed as an argument. Is it ok to write something like the
following?

void foo::bar()
{
add_to_collection(boost::shared_ptr<foo>(this));
}

int main()
{
boost::shared_ptr<foo> f(new foo());
foo->bar();
}

I'm asking because shared_ptr is non-intrusive and I fear
that the above code could create a second counter for an
object that is already reference counted.

Any comments?

 

[Gianni Mariani]

ralpe wrote:
....

I'm asking because shared_ptr is non-intrusive and I fear
that the above code could create a second counter for an
object that is already reference counted.

Any comments?

Use intrusive pointers.

 

[Richard Herring]

Hi,

I have a question regarding boost::shared_ptr. I want to let
shared_ptr take care of my objects' lifetimes, so I change
all my function signatures so that they accept shared_ptrs
instead of raw pointers.

I wonder how to deal with cases where a this pointer is
passed as an argument. Is it ok to write something like the
following?

void foo::bar()
{
add_to_collection(boost::shared_ptr<foo>(this));
}

int main()
{
boost::shared_ptr<foo> f(new foo());
foo->bar();
}

I'm asking because shared_ptr is non-intrusive and I fear
that the above code could create a second counter for an
object that is already reference counted.

Any comments?

You could have a boost::weak_pointer member initialised from this, and
then use boost::make_shared to create a shared_ptr from it to pass to
add_to_collection. Using the weak_ptr will ensure that all shared_ptrs
created from it will share the same reference count.

 

[ralpe]

ralpe wrote:
...

Use intrusive pointers.

I chose shared_ptr because it is going to be standardized.
Will there be an intrusive pointer in the next standard?

 

[tom_usenet]

Hi,

I have a question regarding boost::shared_ptr. I want to let
shared_ptr take care of my objects' lifetimes, so I change
all my function signatures so that they accept shared_ptrs
instead of raw pointers.

I wonder how to deal with cases where a this pointer is
passed as an argument. Is it ok to write something like the
following?

void foo::bar()
{
add_to_collection(boost::shared_ptr<foo>(this));
}

int main()
{
boost::shared_ptr<foo> f(new foo());
foo->bar();
}

I'm asking because shared_ptr is non-intrusive and I fear
that the above code could create a second counter for an
object that is already reference counted.

Any comments?

http://www.boost.org/libs/smart_ptr/sp_techniques.html#from_this

Tom

 

[ralpe]

You could have a boost::weak_pointer member initialised from this, and
then use boost::make_shared to create a shared_ptr from it to pass to
add_to_collection. Using the weak_ptr will ensure that all shared_ptrs
created from it will share the same reference count.

Good idea. Thank you.

Will weak_ptr be part of the next standard?
I only read about shared_ptr.

 

[Simon Turner]

It can indeed.

You could have a boost::weak_pointer member initialised from this, and
then use boost::make_shared to create a shared_ptr from it to pass to
add_to_collection. Using the weak_ptr will ensure that all shared_ptrs
created from it will share the same reference count.

This can be automated by deriving your classes from boost::enable_shared_from_this.

See:
http://www.boost.org/libs/smart_ptr/enable_shared_from_this.html

 

[Richard Herring]

It can indeed.


This can be automated by deriving your classes from
boost::enable_shared_from_this.

See:
http://www.boost.org/libs/smart_ptr/enable_shared_from_this.html

Better still! (enable_... is about two versions of Boost more recent
than my compiler can cope with, so I haven't yet been able to use it
myself :-( )

 

[Marc Schellens]

I know that file functions (see below) are non-standard,
but there must be a 'sub'standard as this are very common
things.

I want to:
get a list of files in a directory,
get the size and access mode of a file, etc.

I am using g++

Thanks,
marc