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+---- Thread: [C] Create object instance based on enumeration value (/showthread.php?tid=5835428)
Create object instance based on enumeration value - chrizator - May 11, 2018
Hey folks, quick question.
Let's assume I have an Enum called 'AnimalType'. And a lot of animals inheriting from a base 'Animal' class (class Dog : public Animal).
Now, based on what 'AnimalType' I have, I want to create the corresponding derivation.
Let's assume (IDK if the shit below compiles, just writing it in HF editor):
Code
enum class AnimalType
{
DogAnimal,
CatAnimal
}
class Animal {}
class Dog : public Animal {}
class Cat : public Animal {}I'd like to have a method that does the following:
Code
Animal* getAnimalFromEnum(AnimalType type)
{
switch (type)
{
case (AnimalType::DogType):
return new Dog()
break;
case (AnimalType::CatType):
return new Cat()
break;
}
}The code above will probably work, but I find it kind of silly. Especially when working with A LOT of Enum values, this is gonna get ugly.
I'd like to have a method using unordered map or something similiar. I don't know how to achieve that. Still new to C++.
Thanks for taking the time! 🤝
RE: Create object instance based on enumeration value - Annabelle Lee - May 11, 2018
Using an unordered map, you can do something like this...
Code
#include <iostream>
#include <functional>
#include <unordered_map>
#include <memory>
enum class AnimalType {
DogAnimal,
CatAnimal
};
// Abstract class. Let's make it require a function that prints type of animal.
class Animal {
public:
virtual void myType() = 0;
};
class Dog : public Animal {
public:
void myType() {
std::cout << "I'm a dog!" << std::endl;
}
};
class Cat : public Animal {
public:
void myType() {
std::cout << "I'm a cat!" << std::endl;
}
};
namespace {
std::unordered_map<AnimalType, std::function<std::unique_ptr<Animal>()>> const animalCreators{
{ AnimalType::DogAnimal, []{ return std::make_unique<Dog>(); } },
{ AnimalType::CatAnimal, []{ return std::make_unique<Cat>(); } },
};
}
std::unique_ptr<Animal> getAnimalFromEnum(AnimalType type) {
return animalCreators.at(type)();
}
int main(int argc, char const *argv[]) {
std::unique_ptr<Animal> myPet = getAnimalFromEnum(AnimalType::DogAnimal);
myPet->myType();
std::unique_ptr<Animal> mySecondPet = getAnimalFromEnum(AnimalType::CatAnimal);
mySecondPet->myType();
return 0;
}RE: Create object instance based on enumeration value - Gorzoid - May 11, 2018
(May 11, 2018 02:46 PM)Annabelle Lee Wrote: Using an unordered map, you can do something like this...
Code#include <iostream>
#include <functional>
#include <unordered_map>
#include <memory>
enum class AnimalType {
DogAnimal,
CatAnimal
};
// Abstract class. Let's make it require a function that prints type of animal.
class Animal {
public:
virtual void myType() = 0;
};
class Dog : public Animal {
public:
void myType() {
std::cout << "I'm a dog!" << std::endl;
}
};
class Cat : public Animal {
public:
void myType() {
std::cout << "I'm a cat!" << std::endl;
}
};
namespace {
std::unordered_map<AnimalType, std::function<std::unique_ptr<Animal>()>> const animalCreators{
{ AnimalType::DogAnimal, []{ return std::make_unique<Dog>(); } },
{ AnimalType::CatAnimal, []{ return std::make_unique<Cat>(); } },
};
}
std::unique_ptr<Animal> getAnimalFromEnum(AnimalType type) {
return animalCreators.at(type)();
}
int main(int argc, char const *argv[]) {
std::unique_ptr<Animal> myPet = getAnimalFromEnum(AnimalType::DogAnimal);
myPet->myType();
std::unique_ptr<Animal> mySecondPet = getAnimalFromEnum(AnimalType::CatAnimal);
mySecondPet->myType();
return 0;
}
+1 unique_ptr, +1 for at(), -1 no virtual destructor doesn't affect your code, but probably would in OP's case
OP, it's pretty much either this or a switch statement, you just gotta pick your poison since there is no runtime reflection of types in C++
RE: Create object instance based on enumeration value - Annabelle Lee - May 11, 2018
(May 11, 2018 04:37 PM)Gorzoid Wrote: +1 unique_ptr, +1 for at(), -1 no virtual destructor doesn't affect your code, but probably would in OP's case
OP, it's pretty much either this or a switch statement, you just gotta pick your poison since there is no runtime reflection of types in C++
Virtual destructor was left out on purpose, since I simply wanted to illustrate the point. Also recently discovered unique_ptr/make_unique and smart pointers, so I've been having a field day playing with them.
RE: Create object instance based on enumeration value - 6577657534735 - May 11, 2018
switch case is better than a map because with all warnings enabled the compiler will warn you if you add a new enum but forget to define a new creation method