problem with default argument with template args
Andrew
template<class A, class B>
class Arg {
public:
Arg() {};
};
template<class A, class B>
class User {
public:
User(const char* username,
Arg<A,B> arg = Arg<A,B>(),
bool flag = false)
{
};
};
gcc fails to compile it. The error output is:
ex2.cpp:12: error: expected `,' or `...' before '>' token
ex2.cpp:12: error: missing `>' to terminate the template argument list
ex2.cpp:12: error: wrong number of template arguments (1, should be 2)
ex2.cpp:3: error: provided for `template<class A, class B> class Arg'
Taking out the defaulting of the template argument makes the problem
go away.
This example, is a cutdown from some legacy code I am looking it. I am
trying to get it to compile with gcc. It compiles with Visual Studio
2005, but then lots of illegal C++ is let through with *that*
compiler.
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Neil Butterworth
> I cannot see what is wrong with the code below:
>
> template<class A, class B>
> class Arg {
> public:
> Arg() {};
> };
>
> template<class A, class B>
> class User {
> public:
> User(const char* username,
> Arg<A,B> arg = Arg<A,B>(),
> bool flag = false)
> {
> };
> };
>
> gcc fails to compile it. The error output is:
>
> ex2.cpp:12: error: expected `,' or `...' before '>' token
> ex2.cpp:12: error: missing `>' to terminate the template argument list
> ex2.cpp:12: error: wrong number of template arguments (1, should be 2)
> ex2.cpp:3: error: provided for `template<class A, class B> class Arg'
>
> Taking out the defaulting of the template argument makes the problem
> go away.
It compiles for me with g++ 4.4.0 - which version of g++ are you using?
Neil Butterworth
Johannes Schaub (litb)
> I cannot see what is wrong with the code below:
>
> template<class A, class B>
> class Arg {
> public:
> Arg() {};
> };
>
> template<class A, class B>
> class User {
> public:
> User(const char* username,
> Arg<A,B> arg = Arg<A,B>(),
> bool flag = false)
> {
> };
> };
>
> gcc fails to compile it. The error output is:
>
> ex2.cpp:12: error: expected `,' or `...' before '>' token
> ex2.cpp:12: error: missing `>' to terminate the template argument list
> ex2.cpp:12: error: wrong number of template arguments (1, should be 2)
> ex2.cpp:3: error: provided for `template<class A, class B> class Arg'
>
> Taking out the defaulting of the template argument makes the problem
> go away.
>
You can fix it with wrapping (...) around the default argument "Arg<A,B>()".
The problem is that in default arguments, names from members of the class
are found even if they're declared afterwards. With older GCC, this
presented a problem that escaped into your code too: GCC always parsed this
as
User(
/* first: param: */ const char* username
/* second param: */ , Arg<A,B> arg = Arg<A,
/* third param: */ , B>(),
/* fourth param: */ , bool flag = false)
See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#325 . It was
fixed in some more recent GCC to compile your code.
Andrew
> Andrew wrote:
> > I cannot see what is wrong with the code below:
>
> > template<class A, class B>
> > class Arg {
> > public:
> > Arg() {};
> > };
>
> > template<class A, class B>
> > class User {
> > public:
> > User(const char* username,
> > Arg<A,B> arg = Arg<A,B>(),
> > bool flag = false)
> > {
> > };
> > };
>
> > gcc fails to compile it. The error output is:
>
> > ex2.cpp:12: error: expected `,' or `...' before '>' token
> > ex2.cpp:12: error: missing `>' to terminate the template argument list
> > ex2.cpp:12: error: wrong number of template arguments (1, should be 2)
> > ex2.cpp:3: error: provided for `template<class A, class B> class Arg'
>
> > Taking out the defaulting of the template argument makes the problem
> > go away.
>
> It compiles for me with g++ 4.4.0 - which version of g++ are you using?
3.4.4-3. I realise this is quite old. It is where gcc is in cygwin at
the moment. AFAIK the only way to get a more recent one is to build it
from the source. I might just do that.
So it looks like the code is legal than. Good. I must admit, I
couldn't see what was wrong with it.
-Andrew M.
Daniel Krügler
> I cannot see what is wrong with the code below:
>
> template<class A, class B>
> class Arg {
> public:
> Arg() {};
>
> };
>
> template<class A, class B>
> class User {
> public:
> User(const char* username,
> Arg<A,B> arg = Arg<A,B>(),
> bool flag = false)
> {
> };
>
> };
>
> gcc fails to compile it. The error output is:
>
> ex2.cpp:12: error: expected `,' or `...' before '>' token
> ex2.cpp:12: error: missing `>' to terminate the template argument list
> ex2.cpp:12: error: wrong number of template arguments (1, should be 2)
> ex2.cpp:3: error: provided for `template<class A, class B> class Arg'
>
> Taking out the defaulting of the template argument makes the problem
> go away.
>
> This example, is a cutdown from some legacy code I am looking it. I am
> trying to get it to compile with gcc. It compiles with Visual Studio
> 2005, but then lots of illegal C++ is let through with *that*
> compiler.
I see neither a problem for theoretical reasons
nor from practical tests with a most recent gcc
4.4.1. Either you are using an older/buggy compiler
or the actual reason for the compiler error
has been eliminated during code simplification.
In my test I instantiated the User template via
User<int, bool> u("");
HTH & Greetings from Bremen,
Daniel Kr�gler
Neil Butterworth
>> It compiles for me with g++ 4.4.0 - which version of g++ are you using?
>
> 3.4.4-3. I realise this is quite old. It is where gcc is in cygwin at
> the moment. AFAIK the only way to get a more recent one is to build it
> from the source. I might just do that.
Or to give up on Cygwin and use the MinGW GCC port instead. MinGW keeps
up with GCC changes pretty well, particularly if you use the Twilight
Dragon branch. I gave up on Cygwin several years ago, using MSYS and
GnuWin32 for tools etc.) and haven't looked back.
Neil Butterworth
Trevor Vaughan
Andrew <marlow...@googlemail.com> wrote:
>> It compiles for me with g++ 4.4.0 - which version of g++ are you using?
>
> 3.4.4-3. I realise this is quite old. It is where gcc is in cygwin at
> the moment. AFAIK the only way to get a more recent one is to build it
> from the source. I might just do that.
On the latest version of GCC under Cygwin, invoking "g++" does give you
version 3.4.4. But, if you invoke "g++-4" instead, you get version 4.3.2.
If you have g++ installed, you probably already have g++-4 installed too.
red floyd
4.1.0 (at least shipped with SLES 10 also has the bug.
stan
>
> Andrew <marlow...@googlemail.com> wrote:
>
>>> It compiles for me with g++ 4.4.0 - which version of g++ are you using?
>>
>> 3.4.4-3. I realise this is quite old. It is where gcc is in cygwin at
>> the moment. AFAIK the only way to get a more recent one is to build it
>> from the source. I might just do that.
>
> On the latest version of GCC under Cygwin, invoking "g++" does give you
> version 3.4.4. But, if you invoke "g++-4" instead, you get version 4.3.2.
> If you have g++ installed, you probably already have g++-4 installed too.
I can't check right now but I don't think Cygwin 1.5 has upgraded past
3.4.4. It's basically frozen while the work goes into 1.7 which does
have both 3 and 4. If you install 1.7, which is pretty stable now, be sure to
read up on "alternatives". That information covers the proper way to
manage GCC 3 vs 4.