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Dan Grose
Guest
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 Posted: Tue Aug 17, 2004 7:22 pm Post subject: Templated member functions - help needed |
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Hi
Cannot get the folowing to compile ( g++ (GCC) 3.2.3 )
template <class T1>
class A
{
public:
template <class T2>
T2* foo() { return new T2; }
};
class B
{
public:
template <class T>
void bar()
{
A<double> a;
a.foo<T>(); // hmmm - does not compile
}
};
int main()
{
A<double> a;
a.foo<int>(); // okay - compiles
return 0;
}
compiler output as follows
g++ mincrim.cpp
mincrim.cpp: In member function `void B::bar()':
mincrim.cpp:19: syntax error before `;' token
Any help much appreciated.
Dan
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Thomas Maeder
Guest
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| Posted: Tue Aug 17, 2004 9:59 pm Post subject: Re: Templated member functions - help needed |
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Dan Grose <grose (AT) wt (DOT) tno.nl> writes:
| Quote: | template <class T1
class A
{
public:
template
T2* foo() { return new T2; }
};
class B
{
public:
template
void bar()
{
A
a.foo<T>(); // hmmm - does not compile
|
a.template foo<T>();
Note that at the time the compiler sees the definition of this member
function, it doesn't know if there will be a specialization of A for double
that will declare the name foo for something different than a member
function template.
Informing the compiler taht foo always names a template allows it to better
parse the code and, potentially, give better diagnostic messages.
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Victor Bazarov
Guest
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| Posted: Tue Aug 17, 2004 10:02 pm Post subject: Re: Templated member functions - help needed |
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Dan Grose wrote:
| Quote: | Cannot get the folowing to compile ( g++ (GCC) 3.2.3 )
template <class T1
class A
{
public:
template
T2* foo() { return new T2; }
};
class B
{
public:
template
void bar()
{
A
a.foo<T>(); // hmmm - does not compile
|
g++ compiles
a.template foo<T>(); // 'template' is needed
(although I am not sure whether it's required by the C++ Standard
or not -- too lazy to look)
| Quote: | }
};
int main()
{
A<double> a;
a.foo<int>(); // okay - compiles
return 0;
}
compiler output as follows
g++ mincrim.cpp
mincrim.cpp: In member function `void B::bar()':
mincrim.cpp:19: syntax error before `;' token
Any help much appreciated.
|
HTH
Victor
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Richard Corden
Guest
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| Posted: Wed Aug 18, 2004 6:14 pm Post subject: Re: Templated member functions - help needed |
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Dan Grose <grose (AT) wt (DOT) tno.nl> writes:
| Quote: | Hi
Cannot get the folowing to compile ( g++ (GCC) 3.2.3 )
template <class T1
class A
{
public:
template
T2* foo() { return new T2; }
};
class B
{
public:
template
void bar()
{
A
a.foo<T>(); // hmmm - does not compile
}
};
int main()
{
A<double> a;
a.foo<int>(); // okay - compiles
return 0;
}
compiler output as follows
g++ mincrim.cpp
mincrim.cpp: In member function `void B::bar()':
mincrim.cpp:19: syntax error before `;' token
|
Although other posters here have mentioned using the 'template'
keyword I don't believe that it is strictly neceesary for the above
example.
14.2/4 says:
When the name of a member template specialization appears after . or
-> in a postfix-expression, or after nested-name-specifier in a
qualified-id, and the postfix-expression or qualified-id explicitly
depends on a template-parameter (14.6.2), the member template name
must be prefixed by the keyword template. Otherwise the name is
assumed to name a non-template.
In the above example 'a' is the postfix-expression and it does not
explicitly depend on a template-parameter. Therefore, we don't need
to add the 'template' keyword.
In more recent revisions of the standard it explicitly states that the
template keyword is allowed where it is not strictly necessary.
However, I do remember a question as to whether the 1998 standard
would allow an unnecessary 'template' keyword, so you might suffer
portability problems adding it for the above example.
Regards,
Richard
--
Richard Corden
To reply remove 's' from address
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James Hopkin
Guest
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| Posted: Wed Aug 18, 2004 6:33 pm Post subject: Re: Templated member functions - help needed |
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Thomas Maeder <maeder (AT) glue (DOT) ch> wrote
| Quote: |
Note that at the time the compiler sees the definition of this member
function, it doesn't know if there will be a specialization of A for double
that will declare the name foo for something different than a member
function template.
|
Actually, 'template' is strictly redundant here. Any subsequent
specialisations of A would make the code illegal.
It appears that explicitly putting template makes the code more
portable to older compilers. For maximum portability, I'd go for
#include <boost/config.hpp>
a.BOOST_NESTED_TEMPLATE foo<T>();
Cheers,
James
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jon hanson
Guest
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| Posted: Fri Aug 20, 2004 12:54 am Post subject: Re: Templated member functions - help needed |
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I experienced a similar error using gcc 3.2.2. In my case the
templated method had an int template parameter and it compiled ok in a
normal function, but failed inside a templated function. If i changed
the method to take an argument i got this error :-
invalid use of member (did you forget the `&' ?)
I think if you change you're foo() method to not be templated it will
compile.
After searching the gcc archives i found some mails which mentioned
this as a bug in gcc.
jon
Dan Grose <grose (AT) wt (DOT) tno.nl> wrote
| Quote: | Hi
Cannot get the folowing to compile ( g++ (GCC) 3.2.3 )
template <class T1
class A
{
public:
template
T2* foo() { return new T2; }
};
class B
{
public:
template
void bar()
{
A
a.foo<T>(); // hmmm - does not compile
}
};
int main()
{
A<double> a;
a.foo<int>(); // okay - compiles
return 0;
}
compiler output as follows
g++ mincrim.cpp
mincrim.cpp: In member function `void B::bar()':
mincrim.cpp:19: syntax error before `;' token
Any help much appreciated.
Dan
|
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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llewelly
Guest
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| Posted: Fri Aug 20, 2004 10:15 am Post subject: Re: Templated member functions - help needed |
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Richard Corden <richards_corden (AT) hotmail (DOT) com> writes:
| Quote: | Dan Grose <grose (AT) wt (DOT) tno.nl> writes:
Hi
Cannot get the folowing to compile ( g++ (GCC) 3.2.3 )
template <class T1
class A
{
public:
template
T2* foo() { return new T2; }
};
class B
{
public:
template
void bar()
{
A
a.foo<T>(); // hmmm - does not compile
}
};
int main()
{
A<double> a;
a.foo<int>(); // okay - compiles
return 0;
}
compiler output as follows
g++ mincrim.cpp
mincrim.cpp: In member function `void B::bar()':
mincrim.cpp:19: syntax error before `;' token
Although other posters here have mentioned using the 'template'
keyword I don't believe that it is strictly neceesary for the above
example.
|
It is not necessary, however, gcc prior to 3.4 offers it as a
work-around for the fact that it cannot correctly parse
'a.foo<T>()' in some examples. gcc 3.4 can parse 'a.foo<T>()'
correctly in all cases, and so no longer requires it.
| Quote: |
14.2/4 says:
When the name of a member template specialization appears after . or
-> in a postfix-expression, or after nested-name-specifier in a
qualified-id, and the postfix-expression or qualified-id explicitly
depends on a template-parameter (14.6.2), the member template name
must be prefixed by the keyword template. Otherwise the name is
assumed to name a non-template.
In the above example 'a' is the postfix-expression and it does not
explicitly depend on a template-parameter. Therefore, we don't need
to add the 'template' keyword.
In more recent revisions of the standard it explicitly states that the
template keyword is allowed where it is not strictly necessary.
However, I do remember a question as to whether the 1998 standard
would allow an unnecessary 'template' keyword, so you might suffer
portability problems adding it for the above example.
|
There are some compilers that reject it. But gcc prior to 3.4
requires it. That's where the money in porting is. :-)
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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