function template compilation error.

[Aman]

Hi,
I'm using the g++ compiler to compile this on SunOS 5.8.
I get compilation errors when I use A* as return type but none when I
used void .
why is this ?
regards,
Aman.
----------errors -------------------

:6: syntax error before `*'
:20: syntax error before `*'
:20: `T' was not declared in this scope
:20: template argument 1 is invalid
:20: `T' was not declared in this scope
:20: parse error before `)'
:21: ANSI C++ forbids declaration `function' with no type

----------- Code --------------------

#include <iostream>
using namespace std ;
template <typename T>
class test {
public :
A* function(T) ; // using void as return type compiles fine .
private :
struct A{
T somedata ;
int i ;
} ;

};
template <typename T>
A* test<T> :: function(T in) // using void as return type works fine.
{
}



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[Jonathan Mcdougall]

Re: function template compilation error.

> Hi,[color=blue]
> I'm using the g++ compiler to compile this on SunOS 5.8.
> I get compilation errors when I use A* as return type but none when I
> used void .
> why is this ?
> regards,
> Aman.
> ----------errors -------------------
>
> :6: syntax error before `*'
> :20: syntax error before `*'
> :20: `T' was not declared in this scope
> :20: template argument 1 is invalid
> :20: `T' was not declared in this scope
> :20: parse error before `)'
> :21: ANSI C++ forbids declaration `function' with no type
>
> ----------- Code --------------------
>
> #include <iostream>
> using namespace std ;
> template <typename T>
> class test {
> public :
> A* function(T) ; // using void as return type compiles fine .
> private :
> struct A{
> T somedata ;
> int i ;
> } ;
>
> };[/color]

Try to put the struct definition before it is used in the class.

[color=blue]
> template <typename T>
> A* test<T> :: function(T in) // using void as return type works fine.[/color]

Should be

template <typename T>
test<T>::A* test<T> :: function(T in)


Jonathan

[Aman]

Re: function template compilation error.

This works fine now , thank you very much .

what's the general rule on using qualification for the types ?
when ever a struct etc uses the typename T , we need qualification with
:: ?

regards,
Aman.


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[Jonathan Mcdougall]

Re: function template compilation error.

> This works fine now , thank you very much .

Acutally, it shouldn't, I missed something :

template <typename T>
test<T>::A* test<T> :: function(T in)

should be

template <typename T>
typename test<T>::A* test<T> :: function(T in)

[color=blue]
> what's the general rule on using qualification for the types ?
> when ever a struct etc uses the typename T , we need qualification with
> :: ?[/color]

In your example

template <typename T>
class test
{
public :
A* function(T);

private :
struct A
{
T somedata ;
int i ;
} ;
};

'A' is an inner class of test, so it must be qualified as 'test::A' and
since 'test' is a template, the syntax is 'test<T>::A', where T could
be anything else.

Furthermore, 'A' is a dependent name of the template so it must be preceeded
by the keywork 'typename' to denote that 'A' is a type. Note that is only
a summary of the rules, you could get C++ Templates by Josuttis and
Vandevoorde (very recommended) to have more informations.


Jonathan

[Aman]

Re: function template compilation error.


hmm..works fine with and without the "typename" !!

Thanks a lot for the summary .
regards,
Aman.


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[Nick Savoiu]

Re: function template compilation error.

Not all compilers complain (yet) about the missing "typename".

Nick

"Aman" <aman@techie.co m> wrote in message
news:576505b7fd 320273de5cc8c1c 5218a54.26421@m ygate.mailgate. org...[color=blue]
>
> hmm..works fine with and without the "typename" !!
>
> Thanks a lot for the summary .
> regards,
> Aman.
>
>
> --
> Posted via Mailgate.ORG Server - http://www.Mailgate.ORG[/color]

[Aman]

Re: function template compilation error.

> Not all compilers complain (yet) about the missing "typename".[color=blue]
>
> Nick[/color]

gotcha !
guess it's a good idea to use it anyway .

thanx
Aman.


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[Jonathan Mcdougall]

Re: function template compilation error.

> > Not all compilers complain (yet) about the missing "typename".

Which is why I missed it.
[color=blue]
> gotcha !
> guess it's a good idea to use it anyway .[/color]

Of course, since your code will eventually break one day and you
won't have the least idea why.


Jonathan