template base other template

[zdun777]

When I try to compile (g++, ver=4.4.5) this code:

#include <map>
using namespace std;

template<typename T> void fun()
{
map<int, int>::iterator b; //OK
map<T, int>::iterator a; //NOK, error: expected ‘;’ before ‘a’
}

,I get follow compiller error:

error: expected ‘;’ before ‘a’

Can you explain me why I can't define iterator in this way ?

 

[Saeed Amrollahi]

When I try to compile (g++, ver=4.4.5) this code:

#include <map>
using namespace std;

template<typename T> void fun()
{
        map<int, int>::iterator b;      //OK
        map<T, int>::iterator a;       //NOK, error: expected ‘;’ before ‘a’

}

,I get follow compiller error:

error: expected ‘;’ before ‘a’

Can you explain me why I can't define iterator in this way ?

Hi
Under g++ 4.5.0, in addition to above
error message, the compiler issues another message, which describes
and gives hint, what you have to do:
error: need 'typename' before std::map<T, int>::iterator' because
'std::map<T, int>'
is a dependent scope.

HTH
-- Saeed Amrollahi

 

[Alain Ketterlin]

#include <map>
using namespace std;

template<typename T> void fun()
{
map<int, int>::iterator b; //OK
map<T, int>::iterator a; //NOK, error: expected ‘;’ before ‘a’
}

,I get follow compiller error:

error: expected ‘;’ before ‘a’

Can you explain me why I can't define iterator in this way ?

The compiler can't guess that "iterator" is a type inside map<T,int>
(you could have a specialization of map for some T where "iterator" is a
static member---and actually that's what the compiler assumes). You have
to say:

typename map<T,int>::iterator a;

-- Alain.

 

[zdun777]

Thanks for explanation.

The compiler can't guess that "iterator" is a type inside map<T,int>
(you could have a specialization of map for some T where "iterator" is a
static member---and actually that's what the compiler assumes). You have
to say:

    typename map<T,int>::iterator a;

-- Alain.