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Frank
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 Posted: Wed Aug 27, 2003 9:46 am Post subject: Creating a template class based on Barton-Nackman trick |
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Hi,
I am learning my ways around using templates in c++. I came across a
piece of code below that shows how the Barton Nackman trick worked.
Then I tried to make a tempalte class out of it. And gcc (version
3.2.2) is not happy about it. I wonder if someone can help me out.
-------------------------------------------------------------------------
#include <vector>
template <class Node_Type>
class Basic_Tree_Node
{
private:
unsigned int name;
Node_Type *ptr2parent;
std::vector<Node_Type*> children;
public:
Basic_Tree_Node(unsigned int node_name,
Node_Type *parent)
:name(node_name), ptr2parent(parent) {}
void add_child(unsigned int name)
{
Node_Type *new_node = new Node_Type(name,
static_cast<Node_Type*>(this));
children.push_back(new_node);
}
};
template <int size>
class Special_Tree_Node1 : public Basic_Tree_Node<Special_Tree_Node1>
{
public:
Special_Tree_Node1(unsigned int name,
Special_Tree_Node1 *ptr2parent)
: Basic_Tree_Node<Special_Tree_Node1>(name, ptr2parent) {}
private:
int sz;
};
int main()
{
Special_Tree_Node1<16> stn1(1, 0);
stn1.add_child(2);
return 0;
}
------------------------------------------------------------------------------
The error messages are:
treenode.cpp:26: type/value mismatch at argument 1 in template
parameter list
for `template<class Node_Type> class Basic_Tree_Node'
treenode.cpp:26: expected a type, got `Special_Tree_Node1'
treenode.cpp: In function `int main()':
treenode.cpp:39: no matching function for call to
`Special_Tree_Node1<16>::
add_child(int)'
treenode.cpp: In constructor
`Special_Tree_Node1<size>::Special_Tree_Node1(unsigned int,
Special_Tree_Node1<size>*) [with int size = 16]':
treenode.cpp:38: instantiated from here
treenode.cpp:30: type `class Basic_Tree_Node<Special_Tree_Node1<16> >'
is not a
direct base of `Special_Tree_Node1<16>'
------------------------------------------------------------------------------
The same code compiled fine on MSVC 6.0. Appreciate any comments.
Frank
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Attila Feher
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| Posted: Wed Aug 27, 2003 9:11 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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Frank wrote:
[SNIP]
| Quote: | template <int size
class Special_Tree_Node1 : public Basic_Tree_Node
[SNIP]
treenode.cpp:26: expected a type, got `Special_Tree_Node1'
[SNIP] |
I might be completely wrong, but as I see it Special_Tree_Node1 is a class
template. A class template is not a type, it will only be a type if
specialized. So as far as I see you will need to write there something
like:
class Special_Tree_Node1 :
public Basic_Tree_Node
But I am not that polished in templates so I might be wrong.
--
Attila aka WW
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ben Hutchings
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| Posted: Thu Aug 28, 2003 10:32 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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In article <f0648a6e.0308261505.47f65990 (AT) posting (DOT) google.com>, Frank wrote:
| Quote: | Hi,
I am learning my ways around using templates in c++. I came across a
piece of code below that shows how the Barton Nackman trick worked.
Then I tried to make a tempalte class out of it. And gcc (version
3.2.2) is not happy about it. I wonder if someone can help me out.
snip
template <int size
class Special_Tree_Node1 : public Basic_Tree_Node
snip
The error messages are:
treenode.cpp:26: type/value mismatch at argument 1 in template
parameter list
for `template
treenode.cpp:26: expected a type, got `Special_Tree_Node1'
snip |
The message is fairly clear! Special_Tree_Node1 is not a type; it's
a template. You need to write Special_Tree_Node1<size> instead.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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johnchx
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| Posted: Thu Aug 28, 2003 10:42 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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[email]an_liqun (AT) yahoo (DOT) com[/email] (Frank) wrote
| Quote: | template <int size
class Special_Tree_Node1 : public Basic_Tree_Node
|
The problem is that Special_Tree_Node1 is the name of a template, not
a type. Try the following:
template
class Special_Tree_Node1:
public Base_Tree_Node< Special_Tree_Node1 {
// ...
};
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ron
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| Posted: Thu Aug 28, 2003 10:44 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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| Quote: | #include <vector
template
class Basic_Tree_Node
{
private:
unsigned int name;
Node_Type *ptr2parent;
std::vector
public:
Basic_Tree_Node(unsigned int node_name,
Node_Type *parent)
:name(node_name), ptr2parent(parent) {}
void add_child(unsigned int name)
{
Node_Type *new_node = new Node_Type(name,
static_cast<Node_Type*>(this));
children.push_back(new_node);
}
};
template
class Special_Tree_Node1 : public Basic_Tree_Node
|
Error xyzzy: Template 'Special_Tree_Node1' requires an argument ^
-- Ron
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Rob Williscroft
Guest
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| Posted: Thu Aug 28, 2003 10:57 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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Frank wrote in news:f0648a6e.0308261505.47f65990 (AT) posting (DOT) google.com:
[snip]
| Quote: | template <int size
class Special_Tree_Node1 : public Basic_Tree_Node
|
class Special_Tree_Node1 :
public Basic_Tree_Node< Special_Tree_Node1< size > >
| Quote: | {
public:
Special_Tree_Node1(unsigned int name,
Special_Tree_Node1 *ptr2parent)
: Basic_Tree_Node<Special_Tree_Node1>(name, ptr2parent) {}
private:
int sz;
};
int main()
{
Special_Tree_Node1<16> stn1(1, 0);
stn1.add_child(2);
return 0;
}
-----------------------------------------------------------------------
-------
The error messages are:
treenode.cpp:26: type/value mismatch at argument 1 in template
parameter list
for `template<class Node_Type> class Basic_Tree_Node'
treenode.cpp:26: expected a type, got `Special_Tree_Node1'
|
Reading template errors can be a bit of an art form, but it
is telling you what you need to know.
Special_Tree_Node1 isn't a type its a class-template, only
when you associate it sutible template paramiters does the name
refer to a type, i.e. Special_Tree_Node1< 16 > is a type.
[snip]
| Quote: |
The same code compiled fine on MSVC 6.0. Appreciate any comments.
|
Upgrade your version of msvc - if you can't give up on the idea
of using any form of 'advanced' template techniques.
Rob.
--
http://www.victim-prime.dsl.pipex.com/
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Dag Henriksson
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| Posted: Thu Aug 28, 2003 8:38 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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"Frank" <an_liqun (AT) yahoo (DOT) com> skrev i meddelandet
news:f0648a6e.0308261505.47f65990 (AT) posting (DOT) google.com...
| Quote: | Hi,
I am learning my ways around using templates in c++. I came across a
piece of code below that shows how the Barton Nackman trick worked.
|
Isn't your code an example of the Curiously Recurring Template Pattern, by
Coplien, and not the Barton-Nackman trick? I thought Barton-Nackman trick
was a technique to inject non-template names into the global scope, by using
a friend function. Once upon a time, when templates could not be overloaded,
this was useful.
--
Dag Henriksson
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Richard Smith
Guest
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| Posted: Fri Aug 29, 2003 8:42 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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Frank wrote:
[heavily truncated]
| Quote: | template <class Node_Type
class Basic_Tree_Node {};
template
class Special_Tree_Node1
: public Basic_Tree_Node
{};
|
As lots of other people have already pointed out, you need
to add template arguments to Special_Tree_Node1. This can
seem quite counter-intuitive given that
template
class Special_Tree_Node1 {
Basic_Tree_Node<Special_Tree_Node1> member;
};
compiles just fine. Similarly, when initialising the base
class, it's perfectly legal to omit the template arguments:
Special_Tree_Node1()
: Basic_Tree_Node<Special_Tree_Node1>()
{}
This has always struck me as somewhat inconsistent, and I've
never really understood why it should be like this.
--
Richard Smith
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Daveed Vandevoorde
Guest
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| Posted: Sat Aug 30, 2003 1:50 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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Richard Smith <richard (AT) ex-parrot (DOT) com> wrote:
| Quote: | Frank wrote:
[heavily truncated]
template <class Node_Type
class Basic_Tree_Node {};
template
class Special_Tree_Node1
: public Basic_Tree_Node
{};
As lots of other people have already pointed out, you need
to add template arguments to Special_Tree_Node1. This can
seem quite counter-intuitive given that
template
class Special_Tree_Node1 {
Basic_Tree_Node
};
compiles just fine. Similarly, when initialising the base
class, it's perfectly legal to omit the template arguments:
Special_Tree_Node1()
: Basic_Tree_Node<Special_Tree_Node1>()
{}
This has always struck me as somewhat inconsistent, and I've
never really understood why it should be like this.
|
Your comments (with which I might agree) prompted me to
investigate this and it is not all that clear to me that the
standard (TC1) makes the original example invalid.
I base this one three references:
14.6.1/1: The injected class name can be used without a
template argument list (the arguments are then implied).
9/2: The injected class name is injected in the class scope.
3.3.6/1: The class scope includes the declarative region
starting after the declaration of the name.
That last part is not entirely clear: I think the text was written
without consideration for implicit declarations. I'll ask for a
Core WG clarification.
Daveed
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Frank
Guest
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| Posted: Sat Aug 30, 2003 7:59 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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[email]an_liqun (AT) yahoo (DOT) com[/email] (Frank) wrote in message news:<f0648a6e.0308261505.47f65990 (AT) posting (DOT) google.com>...
Thanks for all your responses. All you said are correct. But for some
reason, gcc simply won't compile when I have it like this.
template <int size>
class Special_Tree_Node1 : public
Basic_Tree_Node<Special_Tree_Node1
With a lucky stroke, a space was inserted after <size>, and all of a
sudden, it compiles.
template <int size>
class Special_Tree_Node1 : public
Basic_Tree_Node<Special_Tree_Node1
Is there some rule about the space in defining template parameters?
This is totally nuts.
Frank
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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johnchx
Guest
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| Posted: Sun Aug 31, 2003 5:19 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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Richard Smith <richard (AT) ex-parrot (DOT) com> wrote
| Quote: | As lots of other people have already pointed out, you need
to add template arguments to Special_Tree_Node1. This can
seem quite counter-intuitive given that
template <int size
class Special_Tree_Node1 {
Basic_Tree_Node
};
compiles just fine. Similarly, when initialising the base
class, it's perfectly legal to omit the template arguments:
Special_Tree_Node1()
: Basic_Tree_Node<Special_Tree_Node1>()
{}
This has always struck me as somewhat inconsistent, and I've
never really understood why it should be like this.
|
It's a scope thing. ;-)
14.6.1/1 says
Within the scope of a class template, when the name of
the template is neither qualified nor followed by <,
it is equivalent to the name of the template followed
by the template-parameters enclosed in <>.
The base-clause isn't inside the scope of the class, so this rule
doesn't apply there.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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johnchx
Guest
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| Posted: Mon Sep 01, 2003 11:12 am Post subject: Re: Creating a template class based on Barton-Nackman trick |
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[email]google (AT) vandevoorde (DOT) com[/email] (Daveed Vandevoorde) wrote
| Quote: |
3.3.6/1: The class scope includes the declarative region
starting after the declaration of the name.
That last part is not entirely clear: I think the text was written
without consideration for implicit declarations. I'll ask for a
Core WG clarification.
|
3.3.6/1 (1) says:
The potential scope of a name declared in a class consists not only
of the declarative region following the name's declarator....
I don't believe that this can apply to the class name itself because
it has no declarator (thus there is no declarative region following
the name's declarator). The class name appears in the class-head
portion of the class-specifier, which (according to the grammer, A6)
is a type-specifier which is a decl-specifier within a
simple-declaration. No declarator in sight. :-)
Or am I missing something?
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Ben Hutchings
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| Posted: Mon Sep 01, 2003 7:10 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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In article <f0648a6e.0308291546.4ca4bff6 (AT) posting (DOT) google.com>, Frank wrote:
| Quote: | Thanks for all your responses. All you said are correct. But for some
reason, gcc simply won't compile when I have it like this.
template <int size
class Special_Tree_Node1 : public
Basic_Tree_Node
With a lucky stroke, a space was inserted after
sudden, it compiles.
template <int size
class Special_Tree_Node1 : public
Basic_Tree_Node
Is there some rule about the space in defining template parameters?
This is totally nuts.
|
This is a well known problem: the source is tokenised by finding the
longest possible valid token, and ">>" can be a single token, so it is
interpreted as that rather than as two tokens that are end-brackets.
This is not entirely unreasonable, as you could derive from e.g.
Basic_Tree_Node<Special_Tree_Node11> >.
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Daveed Vandevoorde
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| Posted: Tue Sep 09, 2003 12:32 pm Post subject: Re: Creating a template class based on Barton-Nackman trick |
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[email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote:
| Quote: | google (AT) vandevoorde (DOT) com (Daveed Vandevoorde) wrote
3.3.6/1: The class scope includes the declarative region
starting after the declaration of the name.
That last part is not entirely clear: I think the text was written
without consideration for implicit declarations. I'll ask for a
Core WG clarification.
3.3.6/1 (1) says:
The potential scope of a name declared in a class consists not only
of the declarative region following the name's declarator....
I don't believe that this can apply to the class name itself because
it has no declarator (thus there is no declarative region following
the name's declarator).
|
That's what I meant by "not entirely clear". The definition of
class scope does not allow for names declared without a
declarator (including, in this case, implicit declarations).
Surely we meant for E in
struct S {
enum E { e };
// ...
};
to be in the class scope of S. So my guess is that "declarator"
is not the intended term here.
Daveed
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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