C++模板类的分离编译问题

今天在用C++复现数据结构与算法的C代码时, 遇到了关于模板类的问题

问题

当前复现线性表(LinearList), 目前一共三个文件

• LinearList.h: 线性表模板类的声明

  #ifndef LINEARLIST_H
  #define LINEARLIST_H
  
  template <typename T>
  class LinearList
  {
  private:
      T* array;
      int length;
  public:
      LinearList(int length);
      ~LinearList();
  };
  
  #endif
  

• LinearList.cpp: LinearList.h的实现

  #include "LinearList.h"
  #include <iostream>
  
  template <typename T>
  LinearList<T>::LinearList(int length)
  {
      this->array = new T[length];
      this->length = length;
      for (int i = 0; i < length; i++)
      {
          std::cin >> *(array + i);
      }
      std::cout << "Created Successfully!\n";
  }
  
  template <typename T>
  LinearList<T>::~LinearList()
  {
      delete[] this->array;
      this->array = nullptr;
  }
  

• test.cpp: 测试数据结构定义是否正确

  #include <iostream>
  #include "LinearList.h"
  
  int main()
  {
      LinearList<int> A(3);
      return 0;
  }
  

使用g++编译

g++ LinearList.cpp test.cpp -o test

发现报错:

/usr/bin/ld: /tmp/ccnN4cn1.o: in function `main':
test.cpp:(.text+0x29): undefined reference to `LinearList<int>::LinearList(int)'
/usr/bin/ld: test.cpp:(.text+0x75): undefined reference to `LinearList<int>::~LinearList()'
/usr/bin/ld: test.cpp:(.text+0x9b): undefined reference to `LinearList<int>::~LinearList()'
collect2: error: ld returned 1 exit status

解决办法

所有的C++编译器都不支持模板类的分离式编译, 所以只能将模板类的函数实现也写在头文件.h中

• LinearList.h

  #ifndef LINEARLIST_H
  #define LINEARLIST_H
  
  #include <iostream>
  
  template <typename T>
  class LinearList
  {
  private:
      T* array;
      int length;
  public:
      LinearList(int length);
      ~LinearList();
      int get_length();
  };
  
  template <typename T>
  LinearList<T>::LinearList(int length)
  {
      this->array = new T[length];
      this->length = length;
      for (int i = 0; i < length; i++)
      {
          std::cin >> *(array + i);
      }
      std::cout << "Created Successfully!\n";
  }
  
  template <typename T>
  LinearList<T>::~LinearList()
  {
      delete[] this->array;
      this->array = nullptr;
  }
  
  #endif
  

其他注意事项

记得头文件定义类的时候要加如下结构, 作用是防止编译预处理时多次引入导致重复定义

#ifndef XXX
#define XXX

/*
  code here
*/

#endif // end define XXX