Understanding (simple?) C++ Partial Template Specialization

Question

Note: this seems to be a repost of a problem: C++ - Overload templated class method with a partial specilization of that method

I have boiled down a problem I am having with C++ template specialization down to a simple case.

It consists of a simple 2-parameter template class Thing, where I would like to specialize Thing<A,B>::doSomething() for B=int.

#include <cstdio>

// A 3-parameter template class.
template <class A, class B>
class Thing
{
public:
    Thing(A a, B b) : a_(a), b_(b) {}
    B doSomething();
private:
    A a_;
    B b_;
};

// The generic case works as expected.
template <class A, class B>
B Thing<A,B>::doSomething()
{
    return b_;
}

// This specialization does not work!
template <class A>
int Thing<A,int>::doSomething()
{
    return b_+1;
}

int main() {
    // Setup our thing.
    Thing<double,int> thing(1.0,2);
    // This doesn't compile - but works with the generic case.
    printf("Expecting 3, and getting %i\n", thing.doSomething());
    // Clean up.
    return 0;
}

Unfortunately, g++ exits with the error:

partial_specialization.cpp:30: error: invalid use of incomplete type ‘class Thing<A, int>’
partial_specialization.cpp:8: error: declaration of ‘class Thing<A, int>’

The clang++ compiler is a bit more verbose, but has the same problem:

partial_specialization.cpp:30:19: error: nested name specifier 'Thing<A, int>::' for declaration does not
      refer into a class, class template or class template partial specialization
int Thing<A,int>::doSomething()
    ~~~~~~~~~~~~~~^
partial_specialization.cpp:32:12: error: use of undeclared identifier 'b_'
    return b_+1;
           ^
2 errors generated.

I have read and understood that partial template specializations on functions aren't allowed - but I thought I was partially specializing over classes of Thing in this case.

Any ideas?

What I did: A workaround, as determined from the link provided by the accepted answer:

template< class T >
inline T foo( T const & v ) { return v; }

template<>
inline int foo( int const & v ) { return v+1; }

// The generic case works as expected.
template <class A, class B>
B Thing<A,B>::doSomething()
{
    return foo(b_);
}

Answer 1

Partial specialization of a function template, whether it is member function template or stand-alone function template, is not allowed by the Standard:

template<typename T, typename U> void f() {} //okay  - primary template
template<typename T> void f<T,int>() {}      //error - partial specialization
template<> void f<unsigned char,int>() {}    //okay  - full specialization

But you can partially specialize the class template itself. You can do something like this:

template <class A>
class Thing<A,int>  //partial specialization of the class template
{
    //..
    int doSomething();
};

template <class A>
int Thing<A,int>::doSomething()  { /* do whatever you want to do here */ }

Note that when you partially specialize a class template, then the template parameter-list of member function (in its definition outside the class), must match the template parameter list of the class template partial specialization. That means, for the above partial specialization of the class template, you cannot define this:

template <class A>
int Thing<A,double>::doSomething(); //error

Its not allowed, because the template parameter-list in function definition didn't match the template parameter-list of the class template partial specialization. §14.5.4.3/1 from the Standard (2003) says,

The template parameter list of a member of a class template partial specialization shall match the template parameter list of the class template partial specialization.[...]

For more on this, read my answer here:

C++ - Overload templated class method with a partial specilization of that method

---

So what is the solution? Would you partially specialize your class along with all the repetitive work?

A simple solution would be work delegation, instead of partially specializing the class template. Write a stand-alone function template and specialize this as:

template <class B>
B doTheActualSomething(B & b) { return b;  }

template <>
int doTheActualSomething<int>(int & b) { return b + 1; }

And then call this function template from doSomething() member function as:

template <class A, class B>
B Thing<A,B>::doSomething() { return doTheActualSomething<B>(b_); }

---

Since in your particular case, doTheActualSomething needs to know the value of only one member, namely b_, the above solution is fine, as you can pass the value to the function as argument whose type is the template type argument B, and specialization for int is possible being it full-specialization.

But imagine if it needs to access multiple members, type of each depends on the template type argument-list, then defining a stand-alone function template wouldn't solve the problem, because now there will be more than one type argument to the function template, and you cannot partially specialize the function for just, say, one type (as its not allowed).

So in this case you can define a class template instead, which defines a static non-template member function doTheActualSomething. Here is how:

template<typename A, typename B>
struct Worker
{
   B doTheActualSomething(Thing<A,B> *thing)
   {
      return thing->b_;
   }
};

//partial specialization of the class template itself, for B = int
template<typename A>
struct Worker<A,int>
{
   int doTheActualSomething(Thing<A,int> *thing)
   {
      return thing->b_ + 1;
   }
};

Notice that you can use thing pointer to access any member of the class. Of course, if it needs to access private members, then you've to make struct Worker a friend of Thing class template, as:

//forward class template declaration
template<typename T, typename U> struct Worker

template <class A, class B>
class Thing
{
    template<typename T, typename U>  friend struct Worker; //make it friend
   //...
};

Now delegate the work to the friend as:

template <class A, class B>
B Thing<A,B>::doSomething()
{
    return Worker<A,B>::doTheActualSomething(this); //delegate work
}

Two points to be noted here:

• In this solution, doTheActualSomething is not a member function template. Its not enclosing class which is template. Hence we can partially specialize the class template anytime, to get the desired effect of the partial member function template specialization.
• Since we pass this pointer as argument to the function, we can access any member of the class Thing<A,B>, even private members, as Worker<T,U> is also a friend.

Complete online demo : http://www.ideone.com/uEQ4S

---

Now there is still a chance of improvement. Now all instantiations of Worker class template are friends of all instantiation of Thing class template. So we can restrict this many-to-many friendship as:

template <class A, class B>
class Thing
{
    friend struct Worker<A,B>; //make it friend
   //...
};

Now only one instantiation of Worker class template is a friend of one instantiation of Thing class template. That is one-to-one friendship. That is, Worker<A,B> is a friend of Thing<A,B>. Worker<A,B> is NOT a friend of Thing<A,C>.

This change requires us to write the code in somewhat different order. See the complete demo, with all the ordering of class and function definitions and all:

http://www.ideone.com/6a1Ih