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struct Banana : public FruitBase
{
    Banana(Fruit fruit, int price, std::string name, int length):
...
};

struct Apple : public FruitBase
{
    Apple(Fruit fruit, int price, std::string name):
...
};

template<typename... Ts>
std::unique_ptr<FruitBase> makeFruits(Fruit fruit, Ts&&... params)
{
    switch(fruit)
    {
    case Fruit::APPLE:
        return std::make_unique<Apple>(Fruit::APPLE, std::forward<Ts>(params)...);    
        break;
    case Fruit::BANANA:
        return std::make_unique<Banana>(Fruit::BANANA, std::forward<Ts>(params)...);    
        break;    
    }
    return nullptr;
}

int main()
{
    auto apple_ptr = makeFruits(Fruit::APPLE, 10, "gala");
    print_fruit(apple_ptr.get());
    
    auto banana_ptr = makeFruits(Fruit::BANANA, 20, "plantains", 123);
    print_fruit(banana_ptr.get());

}

main.cpp:37:5: note: candidate: 'Apple::Apple(Fruit, int, std::string)'
   37 |     Apple(Fruit fruit, int price, std::string name):
      |     ^~~~~
main.cpp:37:5: note:   candidate expects 3 arguments, 4 provided

main.cpp:26:5: note: candidate: 'Banana::Banana(Fruit, int, std::string, int)'
   26 |     Banana(Fruit fruit, int price, std::string name, int length):
      |     ^~~~~~
main.cpp:26:5: note:   candidate expects 4 arguments, 3 provided

Question> The issue is that the compiler has problems choosing which constructor for Apple and which one for Banana. I assume I could use SFINAE(i.e. with help of std::enable_if) to help compiler deduce the correct function. However, I didn't find an example where the checking is for VALUE instead of TYPE. Can someone give me a little hint here?

CodePudding user response：

Even if not taken, all branches should be valid

If argument is passed as template parameter (so known at compile time), you might do

template<Fruit fruit, typename... Ts>
std::unique_ptr<FruitBase> makeFruits(Ts&&... params)
{
    if constexpr (fruit == Fruit::APPLE)
        return std::make_unique<Apple>(Fruit::APPLE, std::forward<Ts>(params)...);
    else if constexpr (fruit == Fruit::BANANA)
        return std::make_unique<Banana>(Fruit::BANANA, std::forward<Ts>(params)...);
    else
        return nullptr;
}

int main()
{
    auto apple_ptr = makeFruits<Fruit::APPLE>(10, "gala");
    auto banana_ptr = makeFruits<Fruit::BANANA>(20, "plantains", 123);
}

Demo

You can still add SFINAE on top of that.