digitalmars.D.bugs - Is a template like copy-paste?
- Ary Manzana (45/45) Oct 27 2006 I have a template for singleton:
- Chris Nicholson-Sauls (11/74) Oct 27 2006 First off, very nifty to use typeof() in a Singleton template! I never ...
- Sean Kelly (7/70) Oct 27 2006 Templates are like a form of structured macro expansion, but you're
- Bruno Medeiros (6/69) Oct 31 2006 Consider: http://d.puremagic.com/bugzilla/show_bug.cgi?id=49
I have a template for singleton:
---
template Singleton() {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
---
I use it like this:
---
class Bla { // Line 15
mixin Singleton!();
}
void main() {
Bla bla = Bla.instance;
}
---
The compiler says:
main.d(15): class main.Bla main.Bla.Singleton!().this is private
Then I copy-paste the template into the class definition, to see what
happens:
---
class Bla {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
void main() {
Bla bla = Bla.instance;
}
---
Compiles fine.
If I change "private this() { }" to "protected this() { }" in the
singleton template, both compile fine. However, this is just a
workaround, the first should also work.
Oct 27 2006
Chris Nicholson-Sauls <ibisbasenji gmail.com>
Ary Manzana wrote:
I have a template for singleton:
---
template Singleton() {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
---
I use it like this:
---
class Bla { // Line 15
mixin Singleton!();
}
void main() {
Bla bla = Bla.instance;
}
---
The compiler says:
main.d(15): class main.Bla main.Bla.Singleton!().this is private
Then I copy-paste the template into the class definition, to see what
happens:
---
class Bla {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
void main() {
Bla bla = Bla.instance;
}
---
Compiles fine.
If I change "private this() { }" to "protected this() { }" in the
singleton template, both compile fine. However, this is just a
workaround, the first should also work.
First off, very nifty to use typeof() in a Singleton template! I never even
tried that.
Secondly, consider moving the _instance static variable into the instance()
function...
maybe its just me, but I tend to find that cleaner. :) Different strokes,
maybe.
Third, technically no templates aren't quite like copy-paste. The issue is,
mixins have
their own scope. Protected is the way to go; besides, it makes your Singleton
class
derivable. Consider it a bonus feature. Personally, I'd always just put the
instance()
function into the template, and expected the class designer to be intelligent
enough to
make the constructor protected/private, otherwise you're screwed if your
Singleton needs
to do any startup work.
-- Chris Nicholson-Sauls
Oct 27 2006
Ary Manzana wrote:
I have a template for singleton:
---
template Singleton() {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
---
I use it like this:
---
class Bla { // Line 15
mixin Singleton!();
}
void main() {
Bla bla = Bla.instance;
}
---
The compiler says:
main.d(15): class main.Bla main.Bla.Singleton!().this is private
Then I copy-paste the template into the class definition, to see what
happens:
---
class Bla {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
void main() {
Bla bla = Bla.instance;
}
---
Compiles fine.
If I change "private this() { }" to "protected this() { }" in the
singleton template, both compile fine. However, this is just a
workaround, the first should also work.
Templates are like a form of structured macro expansion, but you're
dealing with mixins, not templates per se. mixins work just like
import, so you can consider Singleton to be like its own little module
being imported into the scope of Bla. privates in Singleton, then, are
not visible to Bla.
Sean
Oct 27 2006
Ary Manzana wrote:
I have a template for singleton:
---
template Singleton() {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
---
I use it like this:
---
class Bla { // Line 15
mixin Singleton!();
}
void main() {
Bla bla = Bla.instance;
}
---
The compiler says:
main.d(15): class main.Bla main.Bla.Singleton!().this is private
Then I copy-paste the template into the class definition, to see what
happens:
---
class Bla {
private static typeof(this) _instance;
private this() { }
public static typeof(this) instance() {
if (!_instance) {
_instance = new typeof(this)();
}
return _instance;
}
}
void main() {
Bla bla = Bla.instance;
}
---
Compiles fine.
If I change "private this() { }" to "protected this() { }" in the
singleton template, both compile fine. However, this is just a
workaround, the first should also work.
Consider: http://d.puremagic.com/bugzilla/show_bug.cgi?id=49
(that bug reported is outdated though, it needs to be updated)
--
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D
Oct 31 2006