Re: Do I need a singleton here? Or can I use std::move

James Kanze <>
Thu, 21 Oct 2010 02:57:16 -0700 (PDT)
On Oct 20, 8:09 pm, "Jim Langston" <> wrote:

"James Kanze" <> wrote in message

On Oct 20, 12:26 am, "Jim Langston" <> wrote:

"Jorgen Grahn" <> wrote in message

On Tue, 2010-10-19, Jim Langston wrote:

I am developing a font class for my opengl graphics library
and I came across a quandry. I have a font which is simply
a value for font id and wrapper code to load the font and
kill it. Works fine, then I notice I don't have a virtual
destructor, nor any destructor at all. So I throw in
a virtual destructor and call the code to unload the
glfont. I run the program, now no fonts are visible.

The problem is that my fonts are stored in a map by font
name and the font, and of course std::maps along with most
containers use copy.

Put differently: the problem is that you provided a copy
constructor and/or operator=() for something that wasn't
really copyable. Always consider if those should be disabled
for new classes you write.

Actually: How can I use a non-copyable class in a standard container?

You can't, at least not directly. The most general solution
would be a variant on the Letter/Envelope idiom, using deep copy
(which maintains copy semantics). Alternatively, using
boost::shared_ptr works (but with reference semantics).

The problem with a deep copy is that creating the font is extremely
expensive. I'm loading something like 10,000 chars and it takes noticable
time to load the fonts. If I had to do a deep copy use of the font would be

I susected as much.

I've been thinking about a shared pointer, but I remember that
C++'s implementation of the shared pointer was broken in
places which is why I don't use it, and I don't like using
boost. I understand that 2010 uses 0x's shared pointers which
fix the issues so I may use them, although I think getting the
move to work would be best.

C++ doesn't (yet) have an implementation of shared pointer, and
move isn't present in all compilers yet either. If you do have
access to a compiler which more or less implements C++0x, then
I think that unique_ptr can be put into a vector; that might
solve your solution.

But there might be an even easier solution. You load fonts, but
do you ever unload them? If not, just use a raw pointer, and
forget about the delete.

In your specific case, I imagine that the actual font objects
will be immutable once they've been created and fully
initialized. This simplifies things a lot, since for immutable
objects, you can use either value or reference semantics, with
no change in behavior. Depending on how "clean" you want to be,
I'd either settle for shared_ptr, or wrap the shared_ptr in some
sort of handle, along the lines of the letter/envelope idiom (so
that it looks like an object, and not a pointer, to the client

This is how I've handled this situation in the past, although
with a wrapped naked pointer and not a shared pointer. I may
wind up using this method.

Yes. The classical letter/envelope uses a raw pointer, and deep
copy, since it supports mutable objects; the goal is to have
both polymorphism and value semantics. Since your objects are
immutable once built, the value semantics vs. reference
semantics don't play a role, and there's absolutely no reason to
do the deep copy. If you need to destruct the font objects at
some time, however, you must maintain some sort of reference
count (and ensure that font objects don't point to other font
objects, or if they do, that there are no cycles).

James Kanze

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