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miclin
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 Posted: Thu Sep 25, 2003 8:53 pm Post subject: Template class with template baseclass with member template |
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Hi,
I was wondering if any Gurus out there can tell me why this doesnt work,
and perhaps a way to make it work:
template<typename T>
class B
{
public:
template<int I>
void doStuff(){
}
};
template<typename T>
class C : public B<T>
{
public:
void doOtherStuff(){
doStuff<2>(); //Mysterious parse error from g++ 3.2
}
};
//end code
It works when either C or B or doStuff() isnt paramaterized, but not when
all three are. Sounds like a mystery is afoot.
Long Time Reader First Time Poster (LTRFTP)
Thanks!
Mike Lin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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George Cristea
Guest
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| Posted: Fri Sep 26, 2003 10:35 am Post subject: Re: Template class with template baseclass with member templ |
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This works with gcc 2.95.2.
7 using namespace std;
8
9 template <class T>
10 class B {
11 public:
12 template <int I>
13 void doStuff() {
14 cout << "B
15 }
16
17 };
18
19 template
20 class C : public B<T> {
21 public:
22 void doOtherStuff() {
23 this->template doStuff<2>();
24 }
25
26 };
27
28 class T {
29 };
30
31 int main()
32 {
33 C<T> c;
34 c.doOtherStuff();
35 return 0;
36 }
"miclin" <miclin (AT) interchange (DOT) ubc.ca> wrote
| Quote: | Hi,
I was wondering if any Gurus out there can tell me why this doesnt work,
and perhaps a way to make it work:
template<typename T
class B
{
public:
template
void doStuff(){
}
};
template
class C : public B
{
public:
void doOtherStuff(){
doStuff<2>(); //Mysterious parse error from g++ 3.2
}
};
//end code
It works when either C or B or doStuff() isnt paramaterized, but not when
all three are. Sounds like a mystery is afoot.
|
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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johnchx
Guest
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| Posted: Sat Sep 27, 2003 12:33 am Post subject: Re: Template class with template baseclass with member templ |
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miclin <miclin (AT) interchange (DOT) ubc.ca> wrote
| Quote: | template<typename T
class B
{
public:
template
void doStuff(){
}
};
template
class C : public B
{
public:
void doOtherStuff(){
doStuff<2>(); //Mysterious parse error from g++ 3.2
}
};
|
YATPLUP! ("Yet Another Two-Phase Look-Up Problem") :-)
The compiler can't figure out what to do about the name doStuff<2> for
two reasons:
First, doStuff is a non-dependent name, so the compiler has to look it
up at the point of the template's declaration. Non-dependent names
are *not* looked up in the scope of dependent base classes (like B<T>,
which depends on the template parameter T). So the first thing we
need to do is make doStuff a dependent name, by prefixing either
this-> or the name of the base class to search (B<T>: , like this:
this->doStuff<2>(); // almost fixed
or
B<T>::doStuff<2>(); // almost fixed
Next, now that we've made doStuff a dependent name, the compiler can't
look it up when parsing the template declaration, so it has no way of
determining whether it names a template (i.e. whether doStuff<2> is a
template-id or a typo). So we have to add the template keyword as a
hint:
this->template doStuff<2>(); // works
B<T>::template doStuff<2>(); // works
I prefer the form that explicitly names the base class to be searched,
simply because I think it expresses the intent more clearly (and makes
the compiler enforce that intent). Others seem to prefer the this->
form. They both work fine in this case.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Hyman Rosen
Guest
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| Posted: Sat Sep 27, 2003 12:40 am Post subject: Re: Template class with template baseclass with member templ |
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miclin wrote:
| Quote: | It works when either C or B or doStuff() isnt paramaterized, but not when
all three are. Sounds like a mystery is afoot.
|
Two-phase name lookup. Dependent base classes aren't searched for
non-dependent names in the template definition, so the doStuff in
doOtherStuff is undefined. Say B<T>::template doStuff<2>(); instead.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Siemel Naran
Guest
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| Posted: Sat Sep 27, 2003 12:45 am Post subject: Re: Template class with template baseclass with member templ |
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"miclin" <miclin (AT) interchange (DOT) ubc.ca> wrote in message
| Quote: | template<typename T
class B
{
public:
template
void doStuff(){
}
};
template
class C : public B
{
public:
void doOtherStuff(){
doStuff<2>(); //Mysterious parse error from g++ 3.2
}
};
|
In a template class, when calling a function from base class and the base
class is a template, you have to qualify the base class name. Let us know
if the following works,
void doOtherStuff(){
B<T>::template doStuff<2>();
}
--
+++++++++++
Siemel Naran
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Thomas Mang
Guest
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| Posted: Sat Sep 27, 2003 1:29 am Post subject: Re: Template class with template baseclass with member templ |
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miclin schrieb:
| Quote: | Hi,
I was wondering if any Gurus out there can tell me why this doesnt work,
and perhaps a way to make it work:
template<typename T
class B
{
public:
template
void doStuff(){
}
};
template
class C : public B
{
public:
void doOtherStuff(){
doStuff<2>(); //Mysterious parse error from g++ 3.2
}
};
|
Change the offending line to
this->template doStuff<2>();
and it works fine.
regards,
Thomas
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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