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Dynamically creating an instance of a class from a string containing the class name in C++

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予麋鹿
予麋鹿 2020-12-31 09:00

Lets say I have a base class with 100 children:

class Base { 
  virtual void feed();
  ...   
};
class Child1 : public Base {
  void feed();  //specific proc
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  • 时光取名叫无心
    时光取名叫无心 (楼主)
    2020-12-31 10:04

    I asked a question entitled automatic registration of object creator function with a macro that has the following example program that runs:

    #include 
#include 
#include 

struct Object{ virtual ~Object() {} }; // base type for all objects

struct ObjectFactory {
  static Object* create(const std::string& id) { // creates an object from a string
    const Creators_t::const_iterator iter = static_creators().find(id);
    return iter == static_creators().end() ? 0 : (*iter->second)(); // if found, execute the creator function pointer
  }

 private:
  typedef Object* Creator_t(); // function pointer to create Object
  typedef std::map Creators_t; // map from id to creator
  static Creators_t& static_creators() { static Creators_t s_creators; return s_creators; } // static instance of map
  template struct Register {
    static Object* create() { return new T(); };
    static Creator_t* init_creator(const std::string& id) { return static_creators()[id] = create; }
    static Creator_t* creator;
  };
};

#define REGISTER_TYPE(T, STR) template<> ObjectFactory::Creator_t* ObjectFactory::Register::creator = ObjectFactory::Register::init_creator(STR)

namespace A { struct DerivedA : public Object { DerivedA() { std::cout << "A::DerivedA constructor\n"; } }; }
REGISTER_TYPE(A::DerivedA, "A");

namespace B { struct DerivedB : public Object { DerivedB() { std::cout << "B::DerivedB constructor\n"; } }; }
REGISTER_TYPE(B::DerivedB, "Bee");

namespace C { struct DerivedC : public Object { DerivedC() { std::cout << "C::DerivedC constructor\n"; } }; }
REGISTER_TYPE(C::DerivedC, "sea");

namespace D { struct DerivedD : public Object { DerivedD() { std::cout << "D::DerivedD constructor\n"; } }; }
REGISTER_TYPE(D::DerivedD, "DEE");

int main(void)
{
  delete ObjectFactory::create("A");
  delete ObjectFactory::create("Bee");
  delete ObjectFactory::create("sea");
  delete ObjectFactory::create("DEE");
  return 0;
}

    compile and run output is:

    > g++ example2.cpp && ./a.out
A::DerivedA constructor
B::DerivedB constructor
C::DerivedC constructor
D::DerivedD constructor

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