C++ - Template Specialization & Partial Specialization

Question

I have been looking all over the internet and stackoverflow for a concrete answer but I can't seem to find one. I have to create a generic class and then implement specific functions. My specific instructions were: You need to make use of Template Expression Parameters and Template Class Specialization and Partial Specialization.

I have a template class:

template <class T, int x, int y>
class Z {
    T **array[x][y];
    public:
         Z();
         void print();
         //and other methods
};

I need to:

1) Only Z's where x= 2 and y = 2 need to have a public method void J()

2) For char Z's of x = 2 and y= 2 J will do something; for everything else it does something else

3) For only Z's where T is char will the array be initialized to some value. For everything else it's 0

Naturally, this works:

template<class T, int x, int y>
Z<T,x,y>::Z<T,x,y>() { //initialize to 0 }

But this doesn't:

template<int x, int y>
Z<char,x,y>::Z<char,x,y>() { //initialize to something}

And likewise (assume J exists) this does not work:

template <class T>
void Z<T,2,2>::J() { //something }

My question is:

Is there any simple method for implementing the above items? I need to keep all the other methods in Z. Giving a hint or pointing in the right direction (maybe I missed a question since there are a lot) would be helpful.

Thanks.

Which errors do you have ? What is J ? It is not clear what you want to do. — Synxis
J is some function. What I need to do is listed 1,2,3. My errors are (next comments): — aoi
For Z<char,x,y>::Z<char,x,y>() { //stuff } : "expected constructor, destructor, or type conversion before '<' token" and "expected `;' before '<' token " — aoi
And assuming J exists: void Z<char,x,y>::J() {//stuff} gives the errors: "invalid use of undefined type class Z<char, x, y>'" and "template definition of non-template void Z<char, x, y>::J()' " — aoi

Synxis Synxis · Accepted Answer · 2012-11-18T21:33:50

It seems you want to define only some functions of some specializations : if print() does not change between the char specialization and the general case, it seems that you don't want to redefine it.

// What you want to do (illegal in C++)
template<int,typename T>
struct Z
{
    T myValue;
    Z();
    void print() { /* ... */ }
};

template<int i, typename T>
Z<i,T>::Z() { /* ... */ }

template<int i>
Z<i,char>::Z() { /* ... */ }

However, it does not work like this. Partial or full specializations of class have almost nothing in common, except 'prototype' of template parameters:

// The two following types have only two things related: the template parameter is an int,
// and the second type is a full specialization of the first. There are no relations between
// the content of these 2 types.
template<int> struct A {};
template<> struct A<42> { void work(); };

You have to declare and define each (partial) specialization:

// Fixed example
template<int,typename T>
struct Z
{
    T myValue;
    Z();
    void print() { /* ... */ }
};
template<int i, typename T>
Z<i,T>::Z() { /* ... */ }

// Specialization for <all-ints,char>
template<int i>
struct Z<i,char>
{
    char myValue;
    char othervalue;
    Z();
    void print() { /* Same code than for the general case */ }
};

template<int i>
Z<i,char>::Z() { /* ... */ }

The only way to escape the code duplication is by using inheritance of traits:

// Example with the print function
template<typename T>
struct print_helper
{
    void print() { /* ... */ }
};

// Fixed example
template<int,typename T>
struct Z : public print_helper<T>
{
    T myValue;
    Z();
};
template<int i, typename T>
Z<i,T>::Z() { /* ... */ }

// Specialization for <all-ints,char>
template<int i>
struct Z<i,char> : public print_helper<char>
{
    char myValue;
    char othervalue;
    Z();
};

template<int i>
Z<i,char>::Z() { /* ... */ }

You cannot do what you want without duplication for the moment (the feature removing code duplication is static if and has been proposed for the next C++ standard, see n3322 and n3329).

C++ - Template Specialization & Partial Specialization

2 Answers