CODEWITHSUNDEEP
c++templates
Bartolo Della Corte
Bartolo Della Corte

Reputation: 23

Nested Template Class

I am trying to obtain a nested template class.
The compiler gives me the following error:

error: type/value mismatch at argument 1 in template parameter list for ‘template class MyClass’ note: expected a class template, got ‘SpecificType {aka TemplateClass< Type1, Typ2, Type3 >}

This is my code

/* ...class definition... */
template < template < typename T1,typename T2,typename T3 > typename T>
class MyClass{ 
    // is there a way to have these types available here
    // passing one single specialized type in the main?
    T1 member1;
    T2 member2;
    T3 member3;
    T<T1, T2, T3> member4;
};

/*  ...in my main... */
using SpecificType =  TemplateClass< Type1, Type2, Type3 >;
SpecificType test_object; //until here compiles
MyClass< SpecificType > myclass;

I have already tried to define a SpecificType object, and until that point my code compiles and works.

Upvotes: 2

Views: 453

Answers (1)

Klaus
Klaus

Reputation: 25663

You define MyClass to accept a template as parameter and not a type. As this you simply should pass the template e.g. TemplateClass itself.

So your definition says: I want to have a template which itself has three template parameters. So you must pass a template and not a instance of a template.

/* ...class definition... */
template < template < typename T1,typename T2,typename T3 > typename T>
class MyClass{ /*...*/};

template <typename T1, typename T2, typename T3>
class TemplateClass {};

/*  ...in my main... */
int main()
{
    MyClass< TemplateClass > myclass;
}

After passing the template to your class, inside the class you can use the template to instantiate it like:

template < template < typename T1,typename T2,typename T3 > typename T>
class MyClass
{
    T<int, float, char> obj;
};

After getting your comment, you want a different thing! You want to pass a type, here you want the instance of a template. So simply do the following:

/* ...class definition... */
template < typename T>
class MyClass
{ 
    T obj;
};

template <typename T1, typename T2, typename T3> 
class TemplateClass {}; 

int main()
{   
    MyClass< TemplateClass< int, float, char> > myclass;
}

And now to get the second request from OP :-)

It is possible to pass a type ( e.g. here we see the instantiated template ) and get out the types from the template parameters. Look here:

template < typename T>
class MyClass;

template < template < typename ,typename ,typename  > typename T, typename T1, typename T2, typename T3>
class MyClass< T<T1,T2,T3> >
{
    T1 var1; // here you can use the types... 
    T2 var2;

    T<T1,T2,T3> obj;
};

template <typename T1, typename T2, typename T3>
class TemplateClass {};

int main()
{
    MyClass< TemplateClass< int, float, char> > myclass;
}

Upvotes: 4

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