a template ref class isn't seen in a client project

Question

I have a template class :

namespace MyApp
{
namespace Interop
{

    template < class A, class B>
    public ref class MyClass
    {
        public:
            MyClass(){}
            ~MyClass(){}

    };

}
}

In a client project this class isn't visible:

class A;
class B;
using namespace MyApp::Interop;

void func()
{
    MyClass< A,B >^ h;
}

I get error C2065: 'MyClass' : undeclared identifier. If MyClass isn't template it is visible in this context.

This isn't plain C++. Please retag it with the appropriate language (C++-CLI or C++/CX, whichever it is). — chris, Mar 19 '14 at 20:29
What on earth is is the semicolon doing in `namespace Interop {;` ?? Is this some sort of ancient viking C++? — Omar Himada, Mar 19 '14 at 20:35

score 2 · Accepted Answer · edited May 23 '17 at 11:52

C++ Templates

Class templates have the usual semantics of C++ templates (in other words, you can't you expect CLR clients to "magically" know about and instantiate C++ templates that aren't even available in Assembly Metadata (i.e. Reflection Info)).

The classical C++ solution is going to be explicit instantiation of all the template arguments that you are going to expect this to be used with.

I'll defer to the standard faq items for C++:

Template instantiation details of GCC and MS compilers
Why can templates only be implemented in the header file? (On explicit instantiation)

CLR Generic Types

In case you wanted a generic CLR type (with CLR semantics, that is) use generic instead of template: Generic Classes (C++/CLI). Example:

// generics_instance_fields1.cpp
// compile with: /clr
// Instance fields on generic classes
using namespace System;

generic <typename ItemType>
ref class MyClass {
// Field of the type ItemType:
public :
   ItemType field1;
   // Constructor using a parameter of the type ItemType:
   MyClass(ItemType p) {
     field1 = p; 
   }
};

int main() {
   // Instantiate an instance with an integer field:
   MyClass<int>^ myObj1 = gcnew MyClass<int>(123);
   Console::WriteLine("Integer field = {0}", myObj1->field1);

   // Instantiate an instance with a double field:
   MyClass<double>^ myObj2 = gcnew MyClass<double>(1.23);
   Console::WriteLine("Double field = {0}", myObj2->field1);

   // Instantiate an instance with a String field:
   MyClass<String^>^ myObj3 = gcnew MyClass<String^>("ABC");
   Console::WriteLine("String field = {0}", myObj3->field1);
}

a template ref class isn't seen in a client project

1 Answers1

C++ Templates

CLR Generic Types