partial specified template class with nontype parameter in C++

Question

In following code, I got an compiling error

#include <iostream>

using namespace std;

template <typename T, int I>
class MyClass {
public:
    void func1(){
    cout<<"default: func1"<<endl;
    }
    void func2(){
    cout<<"default: func2"<<endl;
    }
private:
    T haha;
};

template <typename T>
void MyClass<T, 1>::func1(){
    cout<<"special: func1"<<endl;
};



int main()
{
    MyClass<int, 2> intclass;
    intclass.func1();
    intclass.func2();

    MyClass<double, 1> doubleclass;
    doubleclass.func1();
    doubleclass.func2();
    return 0;
}

I got following compiling error:

partialspecifizednontype.cpp:19: error: invalid use of incomplete type 'class MyClass<T, 1>'
partialspecifizednontype.cpp:6: error: declaration of 'class MyClass<T, 1>'

Any specific reason that I can't do this? Is there any workaround to achieve this?

I notice that if my template class don't have first parameter things are OK. (my gcc is 4.2.1)

Following example is OK:

#include <iostream>

using namespace std;

template <int I>
class MyClass {
public:
    void func1(){
    cout<<"default: func1"<<endl;
    }
    void func2(){
    cout<<"default: func2"<<endl;
    }
};

template<>
void MyClass<1>::func1(){
    cout<<"special: func1"<<endl;
};



int main()
{
    MyClass<2> intclass;
    intclass.func1();
    intclass.func2();

    MyClass<1> doubleclass;
    doubleclass.func1();
    doubleclass.func2();
    return 0;
}

Actually, we can. See for example #include using namespace std; template class MyClass { public: void func1(){ cout<<"default: func1"< void MyClass<1>::func1(){ cout<<"special: func1"< intclass; intclass.func1(); intclass.func2(); MyClass<1> doubleclass; doubleclass.func1(); doubleclass.func2(); return 0; } — Yan Zhu, Mar 13 '14 at 21:55
That's not what "partial" means. You *can* indeed specialize function templates explicitly. — Kerrek SB, Mar 13 '14 at 21:57

score 1 · Accepted Answer · answered Mar 13 '14 at 22:20

Member functions of class templates are also function templates in their own right. That means that you can specialize them explicitly (as you do in your second example), but function templates cannot be specialized partially (as you are trying in your first example).

As a workaround, you can specialize the entire class partially:

template <typename T>
class MyClass<T, 1>
{
public:
    void func1() { cout << "special: func1" << endl; };
    // ...
};

You may need to refactor your code to put common code into a base class to make this approach maintainable.

partial specified template class with nontype parameter in C++

1 Answers1