c++ - C++ 中的继承和模板 : Why doesn't the following piece of code compile?

Question

我有一个简单的 C++ 程序，我无法编译它，尽管我尝试在 google 中搜索并尝试阅读有关模板、继承和 vector 的内容，但我没有任何线索知道我犯了什么错误， 谁能帮帮我吗!! 以下是代码:

template <class T>
class Base
{
  public:
  int entries;
};
template <class T>
class Derive : public Base<T *>
{
  public:
  int j;
  void pankaj(){j = entries;}
  void clear();

};
template <class T> void Derive<T>::clear()
{
  int i;
  int j=entries;
};
int main()
{
  Derive b1;
}

我收到以下错误: pankajkk> g++ 示例.cpp

sample.cpp: In member function 'void Derive<T>::pankaj()':
sample.cpp:14: error: 'entries' was not declared in this scope
sample.cpp: In member function 'void Derive<T>::clear()':
sample.cpp:22: error: 'entries' was not declared in this scope
sample.cpp: In function 'int main()':
sample.cpp:26: error: missing template arguments before 'b1'
sample.cpp:26: error: expected `;' before 'b1'

谢谢!!

Answer 1

必须使用this->foo来访问模板基类中的成员变量foo。 You may ask why.

此外，正如老狐狸所解释的那样，在声明变量b1时必须指定类型T。

template <class T>
class Base
{
  public:
  int entries;
};

template <class T>
class Derive : public Base<T *>
{
  public:
  int j;
  void pankaj(){j = this->entries;}
  void clear();
};

template <class T> void Derive<T>::clear()
{
  int i;
  int j=this->entries;
};

int main()
{
  Derive<int> b1;
}

live demo here