Usage of typename

[Dilip]

Folks

I understand that the keyword typename is also used to indicate that a
dependant parameter is a type. However I am a little unsure why I need
typename in this following example:

template<typename K, typename V>
class MyMap
{
public:
typedef typename std::map<K, V>::iterator MyMap_Iterator;
};

why is a typename needed here? what does iterator have to do with the
template parameters K and V? Irrespective of what K and V are am I not
trying to just typedef std::map's iterator? What am I missing?

 

[=?ISO-8859-15?Q?Juli=E1n?= Albo]

public:
typedef typename std::map<K, V>::iterator MyMap_Iterator;
};

why is a typename needed here? what does iterator have to do with the
template parameters K and V? Irrespective of what K and V are am I not
trying to just typedef std::map's iterator? What am I missing?

std::map is a template, not a class, the class that can have a type called
iterator is map <K,V>. And that class is dependent of K and V. For example,
imagine than std::map has an specialization for some concrete K and V you
use, and that specialization has no member called iterator, or has one that
is not a type. May seem strange in the std::map concrete case, but the
rules are generic, it will be very confusing to have different rules for
standard library templates and other templates.

 

[Jakob Bieling]

Folks

I understand that the keyword typename is also used to indicate that a
dependant parameter is a type. However I am a little unsure why I
need typename in this following example:

template<typename K, typename V>
class MyMap
{
public:
typedef typename std::map<K, V>::iterator MyMap_Iterator;
};

why is a typename needed here? what does iterator have to do with the
template parameters K and V? Irrespective of what K and V are am I
not trying to just typedef std::map's iterator? What am I missing?

Afaik, 'iterator' _could_ (just suppose you have not seen the class
definition, yet) also be a function or a static member variable. By
prepending 'typename' you tell the compiler, that 'iterator' really is a
type inside this specialization of the template, so it does not have to
look it up itself.

I am sure there is a more technical and more accurate explanation to
this, like a quote from the Standard. The above is what I use to
determine, if I need 'typename' or not. I got this explanation from this
newsgroup as well, btw. Try googling the groups for my name and
'typename' in the subject.

hth