C++ 继承模板成员的使用方法

Question

I tried to have a class with a template member. The template member must be an inherited class. I have error when I call the class methods. My code is:

#include <iostream>
#include <string>
#include <typeinfo>

class Base
{
public:
    Base (std::string param) {text=param;}
    virtual ~Base() {};
    virtual void print(){std::cout<<text<<std::endl;};

    std::string text;
    };
class Child1 : public Base
{
public:
    Child1(const std::string& param):Base(param){ }
    ~Child1() {}

};
class Child2 : public Base
{
public:
    Child2(const std::string& param):Base(param){ }
    ~Child2() {}

};

template <typename T>
class Adapter
{
public:
    Adapter(){};
    const T* get() const {return value;}
    void setValue(T* rhs) {value=rhs;}    
    void print(){value->print();}
    void test(){std::cout<<"Adapter method"<<std::endl;}
private:
    T* value;

    };

int main() {



  Child1 ex1("name1");
  Child2 ex2("name2");

  Adapter<Base *> adapt;
  Adapter<Child1 *> adapt1;
  Adapter<Child2 *> adapt2;
  
  std::cout << typeid(adapt2).name() << std::endl;
  adapt2.test();
  adapt1.setValue(&ex1);
  adapt1.print();
}

I have these error:

 In function 'int main()':
67:23: error: no matching function for call to 'Adapter<Child1*>::setValue(Child1*)'
67:23: note: candidate is:

47:11: note: void Adapter<T>::setValue(T*) [with T = Child1*]
47:11: note:   no known conversion for argument 1 from 'Child1*' to 'Child1**'
 In instantiation of 'void Adapter<T>::print() [with T = Child1*]':
68:16:   required from here

48:18: error: request for member 'print' in '*((Adapter<Child1*>*)this)->Adapter<Child1*>::value', which is of pointer type 'Child1*' (maybe you meant to use '->' ?)

Could you help me to find my errors?

Answer 1

Could you help me to find my errors?

The first error :

error: cannot convert ‘Child1*’ to ‘Child1**

is because &ex1 is of type Child1* but adapt1 is of type Adapter<Child1*> meaning that the corresponding member function setValue takes a parameter of type T* = Child** . Essentially, you're passing a Child* to setValue while it expects a Child** . That is, the type of the actual argument you are passing does not matches with the parameter type of the member function.

---

The second error :

error: request for member ‘print’ in ‘*((Adapter*)this)->Adapter::value’, which is of pointer type ‘Child1*’

is because the data member value is of type T* = Child** and thus when you wrote:

value->print(); //this is equivalen to (*value).print();

you're trying to call a member function named print on *value which is of type Child1* . But Child1* is a pointer type and has no member functions. Thus you get the second mentioned error.