Q: typename or not typename?

[Jakob Bieling]

Hi,

I recently switched compilers (VC++7.1 to VC++8) and it started
complaining about my use of 'typename' in template classes. So far, I
have never fully understood where I need to put 'typename' and where
not. Please consider the following snippet:

#include <list>

template <typename T>
struct test_case
{
test_case (/* (1) */T::iterator i = (/* (2) */T::iterator ())
{
(/* (3) */T::iterator ii;
}
};

int main (int argc, char* argv [])
{
test_case <std::list <int> > foo;
}

As you can see, I marked three spots. In (1), the compiler requires
me to put 'typename'. In (2) I am not allowed to. In (3) it compiles
with or without.

Why those differences? And what is the reasoning behind requiring it
at all?

Thanks for your time!
--
jb

(reply address in rot13, unscramble first)

[Victor Bazarov]

Jakob Bieling wrote:

I recently switched compilers (VC++7.1 to VC++8) and it started
complaining about my use of 'typename' in template classes. So far, I
have never fully understood where I need to put 'typename' and where
not. Please consider the following snippet:

#include <list>

template <typename T>
struct test_case
{
test_case (/* (1) */T::iterator i = (/* (2) */T::iterator ())
{
(/* (3) */T::iterator ii;
(1): yes, (2): no, (3): yes.
}
};

int main (int argc, char* argv [])
{
test_case <std::list <int> > foo;
}

As you can see, I marked three spots. In (1), the compiler requires
me to put 'typename'. In (2) I am not allowed to. In (3) it compiles
with or without.

Why those differences? And what is the reasoning behind requiring it
at all?

It's required for (1) because you need to help your compiler to understand
that what it's compiling is a declaration (same with (3)). However, since
(2) is the expression, 'typename' has to be omitted.

Imagine that you try to instantiate your 'test_case' template with a class
that has 'iterator' as a function or a data member? _You_ know that it is
not in 'std::list<int>' but when the compiler first looks at the template,
it has no way to know.

V
--
Please remove capital As from my address when replying by mail

[Rolf Magnus]

Jakob Bieling wrote:

Hi,

I recently switched compilers (VC++7.1 to VC++8) and it started
complaining about my use of 'typename' in template classes. So far, I
have never fully understood where I need to put 'typename' and where
not.
You need it if the type depends on a template argument.
Please consider the following snippet:

#include <list>

template <typename T>
struct test_case
{
test_case (/* (1) */T::iterator i = (/* (2) */T::iterator ())
{
(/* (3) */T::iterator ii;
}
};

int main (int argc, char* argv [])
{
test_case <std::list <int> > foo;
}

As you can see, I marked three spots. In (1), the compiler requires
me to put 'typename'. In (2) I am not allowed to. In (3) it compiles
with or without.
Hmm, I would have expected that it's needed in all three places. My compiler
(g++) requires it in (1) and (3) and doesn't care in (2).
Why those differences? And what is the reasoning behind requiring it
at all?

Because in some situations, it's quite some extra work for the compiler to
find out whether T::iterator is the name of a type or a variable, since it
doesn't know anything about T yet when parsing the template. To reduce
context dependancies, 'typename' is used to tell the compiler that the name
denotes a type.