Template class with "typename" - c++

Question

I have a template class where I want to use objects of that class (along with the parameterized type) inside a map. So far this is the solution that I've been able to arrive at:
class IStatMsg;
template <typename T>
class ITier
{
public:
// Methods
ITier(TierType oType) : o_Type(oType){};
virtual ~ITier(){};
typename ITier<T> ParamITier; // line 60
ITier* Get(T oKey)
{
std::map<T, ParamITier*>::iterator it = map_Tiers.find(oKey); // line 64
if (it != map_Tiers.end())
return it->second;
return NULL;
}
void Set(T oKey, ITier* pTier)
{
map_Tiers.insert(pair<T, ParamITier*>(oKey, pTier)); // line 74
}
TierType GetType() { return o_Type; }
protected:
// Methods
// Attributes
std::map<T, ParamITier*> map_Tiers; // line 83
TierType o_Type;
private:
// Methods
// Attributes
};
But when I try to compile this code I get a long list of errors:
/home/gayanm/street/src/QueryServer_NEW/ITier.h:60:
error: expected nested-name-specifier
/home/gayanm/street/src/QueryServer_NEW/ITier.h:60:
error: ITier<T>' specified as
declarator-id
/home/gayanm/street/src/QueryServer_NEW/ITier.h:60:
error: perhaps you wantITier'
for a constructor
/home/gayanm/street/src/QueryServer_NEW/ITier.h:60:
error: two or more data types in
declaration of ITier<T>'
/home/gayanm/street/src/QueryServer_NEW/ITier.h:60:
error: expected;' before
"ParamITier"
/home/gayanm/street/src/QueryServer_NEW/ITier.h:83:
error: ParamITier' was not declared
in this scope
/home/gayanm/street/src/QueryServer_NEW/ITier.h:83:
error: template argument 2 is invalid
/home/gayanm/street/src/QueryServer_NEW/ITier.h:83:
error: template argument 4 is invalid
/home/gayanm/street/src/QueryServer_NEW/ITier.h:83:
error: ISO C++ forbids declaration of
map_Tiers' with no type
/home/gayanm/street/src/QueryServer_NEW/ITier.h:
In member function ITier<T>*
ITier<T>::Get(T)':
/home/gayanm/street/src/QueryServer_NEW/ITier.h:64:
error:ParamITier' undeclared (first
use this function)
/home/gayanm/street/src/QueryServer_NEW/ITier.h:64:
error: (Each undeclared identifier is
reported only once for each function
it appears in.)
/home/gayanm/street/src/QueryServer_NEW/ITier.h:64:
error: template argument 2 is invalid
/home/gayanm/street/src/QueryServer_NEW/ITier.h:64:
error: template argument 4 is invalid
/home/gayanm/street/src/QueryServer_NEW/ITier.h:64:
error: expected ;' before '::' token
/home/gayanm/street/src/QueryServer_NEW/ITier.h:66:
error:it' undeclared (first use this
function)
/home/gayanm/street/src/QueryServer_NEW/ITier.h:66:
error: request for member end' in
((ITier)this)->ITier::map_Tiers',
which is of non-class type int'
/home/gayanm/street/src/QueryServer_NEW/ITier.h:
In member functionvoid
ITier::Set(T, ITier)':
/home/gayanm/street/src/QueryServer_NEW/ITier.h:74:
error: request for member insert' in
((ITier*)this)->ITier::map_Tiers',
which is of non-class type int'
/home/gayanm/street/src/QueryServer_NEW/ITier.h:74:
error:pair' undeclared (first use
this function)
/home/gayanm/street/src/QueryServer_NEW/ITier.h:74:
error: expected primary-expression
before ',' token
/home/gayanm/street/src/QueryServer_NEW/ITier.h:74:
error: ParamITier' undeclared (first
use this function)
/home/gayanm/street/src/QueryServer_NEW/ITier.h:74:
error: expected primary-expression
before '>' token
/home/gayanm/street/src/QueryServer_NEW/ITier.h:
At global scope:
/home/gayanm/street/src/QueryServer_NEW/ITier.h:93:
error: baseITier' with
only non-default constructor in class
without a constructor
/home/gayanm/street/src/QueryServer_NEW/ITier.h:109:
error: expected class-name before '{'
token
Could you please point out how to fix these?
Thank You.

Answer 1

Line 60 does not access a depending name. What you use is ITier<T> of which the compiler knows it's a template given an argument. Instead of typename you want to use typedef ;)
Line 64 does access the depending name iterator which is a type-name, so you have to put typename before std::map. I put the two disambiguations, template and typename on this answer: Disambiguations of dependent names.
Line 74 would be right, if you fix the bug in Line 60, as far as i can see.
Line 83 is alright in itself as far as i can see.

Answer 2

Also, I would recommend that you pass const T& to the functions instead of T, since you cannot be sure (it's a template parameter!) that it'll be a "cheap" copy.

Answer 3

Thanks a lot litb. I was able to fix my code with the guidelines you provided.
class IStatMsg;
template <typename T>
class ITier
{
public:
// Methods
ITier(){};
ITier(TierType oType) : o_Type(oType){};
virtual ~ITier(){};
//typename ITier<T> ParamITier;
ITier<T>* Get(T oKey)
{
typename std::map<T, ITier<T>*>::iterator it = map_Tiers.find(oKey);
if (it != map_Tiers.end())
return it->second;
return NULL;
}
void Set(T oKey, ITier<T>* pTier)
{
map_Tiers.insert(std::pair<T, ITier<T>*>(oKey, pTier));
}
TierType GetType() { return o_Type; }
protected:
// Methods
// Attributes
std::map<T, ITier<T>*> map_Tiers;
TierType o_Type;
private:
// Methods
// Attributes
};