c++ - 将抽象类传递给 i/o 宏

Question

我通过确保继承层次结构中的类实现虚拟读写功能来实现序列化:

class base
{
     ...

     virtual void read(std::istream&)=0;
     virtual void write(std::ostream&) const=0;
     virtual std::string is_a() const;
};

BEGIN_NAMESPACE_1(io)
    SERIALISE(base)
END_NAMESPACE_1

其中宏“SERIALISE”实现了“序列化”和“反序列化”函数的重载，以允许通过基类指针轻松进行 i/o:

#define SERIALISE(TYPE)\
void deserialise( boost::shared_ptr<TYPE>& dat, std::istream& ifs )\
{\
    std::string type;\
    read(type, ifs);\
\
    dat = TYPE::make_##TYPE(type);\
    dat->read(ifs);\
}\
\
void serialise( const boost::shared_ptr<TYPE>& dat, std::ofstream& ofs )\
{\
    write(dat->is_a(), ofs);\
    dat->write(ofs);\
}

但是，如果基类包含纯虚函数，我会收到编译器错误“无法分配抽象类型“base”的对象，因为以下函数在“base”中是纯函数...”，大概是因为编译器试图当类名传递给宏调用时实例化抽象基类。有没有办法挽救这个 I/O 设计？

Answer 1

您不能实例化抽象基类或将其用作函数的值参数。

template explicit shared_ptr(Y * p); Requirements: p must be convertible to T *. Y must be a complete type. The expression delete p must be well-formed, must not invoke undefined behavior, and must not throw exceptions.

Effects: Constructs a shared_ptr that owns the pointer p.

Postconditions: use_count() == 1 && get() == p.

Throws: std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety: If an exception is thrown, delete p is called.

Notes: p must be a pointer to an object that was allocated via a C++ new expression or be 0. The postcondition that use count is 1 holds even if p is 0; invoking delete on a pointer that has a value of 0 is harmless.

在这种情况下，编译器不必将模板 shared_ptr 解析为内部具有 TYPE 类型的值，因为它们永远无法实例化。