c++ - 模板类规范和继承

Question

template<typename T>
class CommonBase
{
};

template<typename T>
class Base : public CommonBase<T>
{
 protected:
  bool flag;
};

template<>
class Base<int> : public CommonBase<int>
{
  Base() {flag = false;}
};

它会导致“flag 未在命名空间中声明”的错误。我哪里做错了？

Answer 1

它是类模板 Base 的显式特化。

你必须在类定义中包含数据成员标志

template<>
class Base<int> : public CommonBase<int>
{
protected:
  bool flag;
public:
  Base() {flag = false;}
};

来自 C++ 20 标准(13.8.3 显式特化)

6. A member of an explicitly specialized class is not implicitly instantiated from the member declaration of the class template; instead, the member of the class template specialization shall itself be explicitly defined if its definition is required. ... The definition of an explicitly specialized class is unrelated to the definition of a generated specialization. That is, its members need not have the same names, types, etc. as the members of a generated specialization. Members of an explicitly specialized class template are defined in the same manner as members of normal classes, and not using the template<> syntax....