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Partial Specialization of Class Templates (C++)

Class templates can be partially specialized, and the resulting class is still a template. Partial specialization allows template code to be partially customized for specific types in situations, such as:

  • A template has multiple types and only some of them need to be specialized. The result is a template parameterized on the remaining types.

  • A template has only one type, but a specialization is needed for pointer, reference, pointer to member, or function pointer types. The specialization itself is still a template on the type pointed to or referenced.

// partial_specialization_of_class_templates.cpp
template <class T> struct PTS {
   enum {
      IsPointer = 0,
      IsPointerToDataMember = 0
   };
};

template <class T> struct PTS<T*> {
   enum {
      IsPointer = 1,
      IsPointerToDataMember = 0
   };
};

template <class T, class U> struct PTS<T U::*> {
   enum {
      IsPointer = 0,
      IsPointerToDataMember = 1
   };
};

struct S{};

extern "C" int printf_s(const char*,...);

int main() {
   S s, *pS;
   int S::*ptm;
   printf_s("PTS<S>::IsPointer == %d PTS<S>::IsPointerToDataMember == %d\n", 
           PTS<S>::IsPointer, PTS<S>:: IsPointerToDataMember);
   printf_s("PTS<S*>::IsPointer == %d PTS<S*>::IsPointerToDataMember ==%d\n"
           , PTS<S*>::IsPointer, PTS<S*>:: IsPointerToDataMember);
   printf_s("PTS<int S::*>::IsPointer == %d PTS"
           "<int S::*>::IsPointerToDataMember == %d\n", 
           PTS<int S::*>::IsPointer, PTS<int S::*>:: 
           IsPointerToDataMember);
}
PTS<S>::IsPointer == 0 PTS<S>::IsPointerToDataMember == 0 PTS<S*>::IsPointer == 1 PTS<S*>::IsPointerToDataMember ==0 PTS<int S::*>::IsPointer == 0 PTS<int S::*>::IsPointerToDataMember == 1

If you have a template collection class that takes any type T, you can create a partial specialization that takes any pointer type T*. The following code demonstrates a collection class template Bag and a partial specialization for pointer types in which the collection dereferences the pointer types before copying them to the array. The collection then stores the values that are pointed to. With the original template, only the pointers themselves would have been stored in the collection, leaving the data vulnerable to deletion or modification. In this special pointer version of the collection, code to check for a null pointer in the add method is added.

// partial_specialization_of_class_templates2.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;

// Original template collection class.
template <class T> class Bag {
   T* elem;
   int size;
   int max_size;

public:
   Bag() : elem(0), size(0), max_size(1) {}
   void add(T t) {
      T* tmp;
      if (size + 1 >= max_size) {
         max_size *= 2;
         tmp = new T [max_size];
         for (int i = 0; i < size; i++)
            tmp[i] = elem[i];
         tmp[size++] = t;
         delete[] elem;
         elem = tmp;
      }
      else
         elem[size++] = t;
   }

   void print() {
      for (int i = 0; i < size; i++)
         cout << elem[i] << " ";
      cout << endl;
   }
};

// Template partial specialization for pointer types.
// The collection has been modified to check for NULL 
// and store types pointed to.
template <class T> class Bag<T*> {
   T* elem;
   int size;
   int max_size;

public:
   Bag() : elem(0), size(0), max_size(1) {}
   void add(T* t) {
      T* tmp;
      if (t == NULL) {   // Check for NULL
         cout << "Null pointer!" << endl;
         return;
      }

      if (size + 1 >= max_size) {
         max_size *= 2;
         tmp = new T [max_size];
         for (int i = 0; i < size; i++)
            tmp[i] = elem[i];
         tmp[size++] = *t;  // Dereference
         delete[] elem;
         elem = tmp;
      }
      else
         elem[size++] = *t; // Dereference
   }

   void print() {
      for (int i = 0; i < size; i++)
         cout << elem[i] << " ";
      cout << endl;
   }
};

int main() {
   Bag<int> xi;
   Bag<char> xc;
   Bag<int*> xp; // Uses partial specialization for pointer types.

   xi.add(10);
   xi.add(9);
   xi.add(8);
   xi.print();

   xc.add('a');
   xc.add('b');
   xc.add('c');
   xc.print();

   int i = 3, j = 87, *p = new int[2];
   *p = 8;
   *(p + 1) = 100;
   xp.add(&i);
   xp.add(&j);
   xp.add(p);
   xp.add(p + 1);
   p = NULL;
   xp.add(p);
   xp.print();
}
10 9 8 a b c Null pointer! 3 87 8 100

The following example defines a template class that takes pairs of any two types and then defines a partial specialization of that template class specialized so that one of the types is int. The specialization defines an additional sort method that implements a simple bubble sort based on the integer.

// partial_specialization_of_class_templates3.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;

template <class Key, class Value> class Dictionary {
   Key* keys;
   Value* values;
   int size;
   int max_size;
public:
   Dictionary(int initial_size) :  size(0) {
      max_size = 1;
      while (initial_size >= max_size)
         max_size *= 2;
      keys = new Key[max_size];
      values = new Value[max_size];
   }
   void add(Key key, Value value) {
      Key* tmpKey;
      Value* tmpVal;
      if (size + 1 >= max_size) {
         max_size *= 2;
         tmpKey = new Key [max_size];
         tmpVal = new Value [max_size];
         for (int i = 0; i < size; i++) {
            tmpKey[i] = keys[i];
            tmpVal[i] = values[i];
         }
         tmpKey[size] = key;
         tmpVal[size] = value;
         delete[] keys;
         delete[] values;
         keys = tmpKey;
         values = tmpVal;
      }
      else {
         keys[size] = key;
         values[size] = value;
      }
      size++;
   }

   void print() {
      for (int i = 0; i < size; i++)
         cout << "{" << keys[i] << ", " << values[i] << "}" << endl;
   }
};

// Template partial specialization: Key is specified to be int.
template <class Value> class Dictionary<int, Value> {
   int* keys;
   Value* values;
   int size;
   int max_size;
public:
   Dictionary(int initial_size) :  size(0) {
      max_size = 1;
      while (initial_size >= max_size)
         max_size *= 2;
      keys = new int[max_size];
      values = new Value[max_size];
   }
   void add(int key, Value value) {
      int* tmpKey;
      Value* tmpVal;
      if (size + 1 >= max_size) {
         max_size *= 2;
         tmpKey = new int [max_size];
         tmpVal = new Value [max_size];
         for (int i = 0; i < size; i++) {
            tmpKey[i] = keys[i];
            tmpVal[i] = values[i];
         }
         tmpKey[size] = key;
         tmpVal[size] = value;
         delete[] keys;
         delete[] values;
         keys = tmpKey;
         values = tmpVal;
      }
      else {
         keys[size] = key;
         values[size] = value;
      }
      size++;
   }

   void sort() {
      // Sort method is defined.
      int smallest = 0;
      for (int i = 0; i < size - 1; i++) {
         for (int j = i; j < size; j++) {
            if (keys[j] < keys[smallest])
               smallest = j;
         }
         swap(keys[i], keys[smallest]);
         swap(values[i], values[smallest]);
      }
   }

   void print() {
      for (int i = 0; i < size; i++)
         cout << "{" << keys[i] << ", " << values[i] << "}" << endl;
   }
};

int main() {
   Dictionary<char*, char*>* dict = new Dictionary<char*, char*>(10);
   dict->print();
   dict->add("apple", "fruit");
   dict->add("banana", "fruit");
   dict->add("dog", "animal");
   dict->print();

   Dictionary<int, char*>* dict_specialized = new Dictionary<int, char*>(10);
   dict_specialized->print();
   dict_specialized->add(100, "apple");
   dict_specialized->add(101, "banana");
   dict_specialized->add(103, "dog");
   dict_specialized->add(89, "cat");
   dict_specialized->print();
   dict_specialized->sort();
   cout << endl << "Sorted list:" << endl;
   dict_specialized->print();
}
{apple, fruit} {banana, fruit} {dog, animal} {100, apple} {101, banana} {103, dog} {89, cat} Sorted list: {89, cat} {100, apple} {101, banana} {103, dog}

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