类模板的部分专用化 (C++)

类模板可以部分专用化,生成的类仍是模板。 在类似于下面的情况下,部分专用化允许为特定类型部分自定义模板代码:

  • 模板有多个类型,且只有一部分需要专用化。 结果是基于其余类型参数化的模板。

  • 模板只有一个类型,但指针、引用、指向成员的指针或函数指针类型需要专用化。 专用化本身仍是指向或引用的类型上的模板。

示例

// 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);
}
  

如果有一个采用任何 T 类型的模板集合类,则可以创建采用任何指针类型 T* 的部分专用化。 以下代码演示了一个集合类模板 Bag 以及指针类型的部分专用化,在此专用化中,该集合在将指针类型复制到数组前取消引用它们。 该集合随后存储指向的值。 对于原始模板,只有指针本身将存储在集合中,从而使数据易受删除或修改。 在此特殊指针版本的集合中,添加了在 add 方法中检查 null 指针的代码。

// 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();
}
  

以下示例定义一个采用由任意两种类型构成的对的模板类,然后定义已专用化的模板类的部分专用化,以便让其中一个类型为 int。 该专用化定义了基于整数实现简单气泡排序的另一种排序方法。

// 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();
}
  

请参见

参考

类模板的显式专用化