類似的問題還有： why can't class template use Handle Class Pattern to hide its implementation? || why there are linker problems (undefined reference) to my class template?

我出現(xiàn)問題的源碼（見main.cpp,Stack.h,Stack.cpp）（本來是準(zhǔn)備用來展示Handle Class Pattern如何實現(xiàn)implementation-hiding），報錯如下：

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問題的本質(zhì)&解決辦法：

http://stackoverflow.com/questions/8752837/undefined-reference-to-template-class-constructor

http://www.parashift.com/c++-faq-lite/templates-defn-vs-decl.html

http://stackoverflow.com/questions/5417465/separating-template-interface-and-implementation-in-c

http://stackoverflow.com/questions/18121071/hiding-template-implementation-details-from-doxygen

方法1：Explicitly instantiate the template, and its member definitions

方法2：Copy the implemtation code of the class template into its header file

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總結(jié)：

雖然有2種解決辦法，但是方法一顯然“太笨”，而且“太不靈活”

So, if you plan to create your own class template, then you just don't need to consider enforcing implementation hiding as you do to normal classes, the only way?to hide the implementation of ?a class template is not to provide its header.?

On the other hand, if you decide to design something to be a class template, you must be sure there's nothing need to be hidden for that template, for example:?encryption algorithm or other sensitive stuff.

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Insight Comment：

The very goal of template is to create a "pattern" so that the compiler can generate classes and functions for a multitude of unrelated types. If you hide this pattern, how do you expect the compiler to be able to generate those classes and functions ?

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代碼：

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main.cpp

1 #include "Stack.h" 2 3 #include <iostream> 4 5 using namespace std; 6 7 class Box { 8 public: 9 Box():data(0), ID(num++) { cout << "Box" << ID << " cons" << endl; } 10 Box(const Box &copy): data(copy.data), ID(num++) { cout << "Box" << ID << " copy cons" << endl; } 11 ~Box() { cout << "Box" << ID << " des" << endl; } 12 int data; 13 private: 14 static int num; 15 const int ID; 16 }; 17 18 int Box::num = 1; 19 20 int main() 21 { 22 Box b1,b2,b3; 23 Stack<Box> bstack; 24 bstack.push(b1); 25 bstack.push(b2); 26 bstack.push(b3); 27 return 0; 28 }

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Stack.h

1 #ifndef STACK_H 2 #define STACK_H 3 4 #include <cstddef> 5 6 template <typename T> 7 class StackImpl; // Stack implementation (hidden), private part 8 // will not be seen by clients 9 10 template <typename T> 11 class Stack 12 { 13 public: 14 Stack(); 15 ~Stack(); 16 /** 17 Inserts a new element at the top of the stack, 18 above its current top element. 19 The content of this new element is 20 initialized to a copy of val. 21 @param val value to which the inserted element is initialized 22 */ 23 void push(const T &val); 24 /** 25 @return a reference to the top element in the stack 26 */ 27 T& top(); 28 /** 29 @return a const reference to the top element in the stack 30 */ 31 const T& top() const; 32 /** 33 Removes the element on top of the stack. 34 This calls the removed element's destructor. 35 */ 36 void pop(); 37 /** 38 @return the number of elements in the stack. 39 */ 40 size_t size(); 41 private: 42 43 StackImpl<T> *impl; // Stack implementation (hidden), private part 44 // will not be seen by clients 45 46 }; 47 48 #endif // STACK_H

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Stack.cpp

1 #include "Stack.h" 2 3 #include <stdexcept> 4 5 using namespace std; 6 7 template <typename T> 8 class Link { 9 public: 10 11 T data; 12 Link *next; 13 14 Link(const T &_data): data(_data), next(NULL) {} 15 Link(const T &_data, Link *_next): data(_data), next(_next) {} 16 ~Link() { 17 next = NULL; 18 } 19 20 }; 21 22 template <typename T> 23 class StackImpl { 24 public: // even though they're public, but they're not in the header, thus it's safe 25 26 Link<T> *head; 27 28 size_t size; 29 30 StackImpl(): head(NULL) {} 31 ~StackImpl() { 32 Link<T> *ptr = head; 33 while (ptr != NULL) { 34 ptr = head->next; 35 delete head; 36 head = ptr; 37 } 38 size = 0; 39 } 40 }; 41 42 template <typename T> 43 Stack<T>::Stack(): impl(new StackImpl<T>()) {} 44 45 template <typename T> 46 Stack<T>::~Stack() { 47 if (impl != NULL) 48 delete impl; 49 } 50 /** 51 Inserts a new element at the top of the stack, 52 above its current top element. 53 The content of this new element is 54 initialized to a copy of val. 55 @param val value to which the inserted element is initialized 56 */ 57 template <typename T> 58 void Stack<T>::push(const T &val) 59 { 60 impl->head = new Link<T>(val, impl->head); 61 ++(impl->size); 62 } 63 /** 64 @return a reference to the top element in the stack 65 */ 66 template <typename T> 67 T& Stack<T>::top() 68 { 69 if (impl->head == NULL) 70 throw runtime_error("empty stack"); 71 return impl->head->data; 72 73 } 74 /** 75 @return a const reference to the top element in the stack 76 */ 77 template <typename T> 78 const T& Stack<T>::top() const 79 { 80 if (impl->head == NULL) 81 throw runtime_error("empty stack"); 82 return impl->head->data; 83 } 84 /** 85 Removes the element on top of the stack. 86 This calls the removed element's destructor. 87 */ 88 template <typename T> 89 void Stack<T>::pop() 90 { 91 if (impl->head == NULL) 92 throw runtime_error("empty stack"); 93 Link<T> *ptr = impl->head->next; 94 delete impl->head; 95 impl->head = ptr; 96 --(impl->size); 97 } 98 99 /** 100 @return the number of elements in the stack. 101 */ 102 template <typename T> 103 size_t Stack<T>::size() { 104 return impl->size; 105 }

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轉(zhuǎn)載于:https://www.cnblogs.com/qrlozte/p/4108807.html

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