Re: Template Problem
On 11 Sep., 14:09, Michael Adams <ondelette2...@gmail.com> wrote:
Unfortunately, these two cases
do not compile successfully, and both produce a similar error message.
In particular, I obtain the following error messages:
if (TESTCASE == 1), I get:
foo5.cc:49: error: 'pointer' does not name a type
if (TESTCASE == 2), I also get:
foo5.cc:49: error: 'pointer' does not name a type
I would appreciate very much if someone could explain to me what I am
doing wrong (i.e., why the code fails to compile) and how I might
resolve this problem.
The error is easy to explain: For better explanation let's look into
your template:
class Vertex : public VertexBase
{
public:
int someExtraInfo;
};
template <class V>
class Mesh
{
public:
typedef V Vertex;
class VertexIterator : public
std::iterator<std::bidirectional_iterator_tag,
V,
ptrdiff_t>
{
private:
pointer vertex_;
};
};
The type "pointer" inside VertexIterator will not be found,
because name lookup of unqualified names does not look into
base classes of templates, see 14.6.2/3:
"In the definition of a class template or a member of a class
template, if a base class of the class template depends on a
template-parameter, the base class scope is not examined
during unqualified name lookup either at the point of definition
of the class template or member or during an instantiation of
the class template or member."
All conditions are fulfilled: Both V and Vertex are dependent,
because V is the template parameter and Vertex is just a typedef
for that. The template std::iterator is used as the base class,
and depends also on V, so the lookup wont consider the internals
of std::iterator (This is for good reason, if you consider the
fact, that templates can be specialized, so we might have end up
in an instantiation, which does not have any type "pointer" at
all). And the simple fix is to use a *qualified* name. Starting
with a naive approach, we would try this:
template <class V>
class Mesh
{
public:
typedef V Vertex;
class VertexIterator : public
std::iterator<std::bidirectional_iterator_tag,
V,
ptrdiff_t>
{
private:
VertexIterator::pointer vertex_;
};
};
This will alos no compile, because the compiler
does not know whether VertexIterator::pointer names
a type or not. The solution is given by 14.6/3:
"A qualified-id that refers to a type and in which the
nested-name-specifier depends on a template-parameter
(14.6.2) shall be prefixed by the keyword typename to
indicate that the qualified-id denotes a type, forming
an elaborated-type-specifier (7.1.5.3)."
So the answer to the problem is:
template <class V>
class Mesh
{
public:
typedef V Vertex;
class VertexIterator : public
std::iterator<std::bidirectional_iterator_tag,
V,
ptrdiff_t>
{
private:
typename VertexIterator::pointer vertex_;
};
};
Greetings from Bremen,
Daniel Kr?gler
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