Access template parameter from class object

user_6391417 0
Viewed 2728

I have a class template in myclass.hpp: 

template<class T, class P>
class myclass
{
....
};

In my main.cc I create an object of the class:

myclass<int, double> mc;
otherfunc<myclass>(mc);

In some other header file header1.hpp:

template<class MyClass>
void otherfunc(MyClass const &mc)
{
/* Access through 'mc' the underlying template parameters T and P*/
}

How can I access the template parameter T and P in header1.hpp?

class-template templates c++
3 Answers

How can I access the template parameter T and P in header1.hpp?

Provide public type definitions in your class myclass:

template<class T, class P>
class myclass
{
public:
     typedef T T_type;
     typedef P P_type;
....
};

Thus you can access those types as 

typename myclass::T_Type x;
typename myclass::P_Type y;

elsewhere.

user_14133950

Example:

template<class T, class P>
void otherfunc(myclass<T, P> const &mc)
{}

Alternatively:

template<class T, class P>
class myclass
{
    using ParamT = T;
    using ParamP = P;
};

template<class MyClass>
void otherfunc(MyClass const &mc)
{
    using ParamT = typename MyClass::ParamT;
    using ParamP = typename MyClass::ParamP;
}
user_4120800

#1

One way is to typedef within myclass.

template<class T, class P>
class myclass
{
public:
    typedef T typeT;
    typedef P typeP;
};

And refer to them like

template<class MyClass>
void otherfunc(MyClass const &mc)
{
    typename MyClass::typeT myMember;
}

#2

Another way is to use decltype. You likely don't literally need to use the template parameters, but intend to use the same type as a member or return value of a myclass member. Thus, something like this:

template<class T, class P>
struct myclass
{
  T memberT;
  P memberP;
};

template<class MyClass>
void otherfunc(MyClass const &mc)
{
  using T = decltype(MyClass::memberT);
  using P = decltype(MyClass::memberP);
  T var1;
  P var2;
}
user_4021690
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