c++ - 因为以下虚函数在一个类中是纯的

Question

我正在尝试编译以下代码。起初一切都很好。我做了一些修改，然后出现以下错误。我真的真的不明白发生了什么:

main.cpp: In function 'int main()':
main.cpp:38:32: error: cannot declare variable 'mp' to be of abstract type 'MasterPricerContentManager'
     MasterPricerContentManager mp(&query);
                                ^
main.cpp:20:7: note:   because the following virtual functions are pure within 'MasterPricerContentManager':
 class MasterPricerContentManager: public ContentManager {
       ^
main.cpp:17:24: note:   virtual std::string* ContentManager::getAnswer()
   virtual std::string *getAnswer()   = 0;

据我了解，如果一个函数是纯虚函数，我必须在派生类中实现它。为什么我会遇到这个问题？

---

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#include <iostream>

using namespace std;

class ContentManager {
 public:
  virtual bool execute()= 0;
  virtual void setQuery(std::string *query) = 0;
  virtual std::string *getAnswer()   = 0;
};

class MasterPricerContentManager: public ContentManager {
 public:
   explicit MasterPricerContentManager(std::string *query):_query(query) {}
    ~MasterPricerContentManager() {}
    MasterPricerContentManager(const MasterPricerContentManager &)= delete;

    bool execute() {return true;}
    void setQuery(std::string  *query) {_query = query; }
    std::string       *getAnswer() const { return {}; }

  private:
    std::string * _query;

};

int main()
{  
    std::string query = "hello";
    MasterPricerContentManager mp(&query);

    return 0;
}

Answer 1

why i am getting that problem?

因为你的基类中的签名与你的子类中的不同:

 virtual std::string *getAnswer()   = 0;

对比

 std::string       *getAnswer() const { return {}; }

您需要将 const 修饰符添加到基类中的声明中，或者将其从子类中的覆盖中移除。