How to select a class member using templates

Question

Consider the example code below.

#include <iostream>

enum class FruitType {
    APPLE,
    ORANGE
};

class Fruit {
    uint64_t apple_count;
    uint64_t orange_count;

public:
    Fruit() : apple_count(0), orange_count(0) {}
    void increase_count(FruitType type) {
        if (type == FruitType::APPLE)
            apple_count++;
        else
            orange_count++;
    }
    void print() {
        std::cout << "apple_count: " << apple_count << std::endl;
        std::cout << "orange_count: " << orange_count << std::endl;
    }
};

int main(int, char**) {
    Fruit fruit;
    fruit.increase_count(FruitType::APPLE);
    fruit.increase_count(FruitType::ORANGE);
    fruit.increase_count(FruitType::APPLE);
    fruit.increase_count(FruitType::APPLE);
    fruit.increase_count(FruitType::ORANGE);
    fruit.print();

    return 0;
}

I want the increase_count function to effectively compile to increase_count_apple and increase_count_orange. This is a simplified example of another class where such compile-time optimisation helps me at runtime, in theory. Intuitively, this feels possible using templates, but I am unable to arrive at a satisfactory output.

Answer 1

By making increase_count a template that takes a non-type template paramter you can do

class Fruit {
    uint64_t apple_count;
    uint64_t orange_count;

public:
    Fruit() : apple_count(0), orange_count(0) {}
    template <FruitType type>
    void increase_count() {
        if constexpr (type == FruitType::APPLE)
            apple_count++;
        else
            orange_count++;
    }
    void print() {
        std::cout << "apple_count: " << apple_count << std::endl;
        std::cout << "orange_count: " << orange_count << std::endl;
    }
};

int main(int, char**) {
    Fruit fruit;
    fruit.increase_count<FruitType::APPLE>();
    fruit.increase_count<FruitType::ORANGE>();
    fruit.increase_count<FruitType::APPLE>();
    fruit.increase_count<FruitType::APPLE>();
    fruit.increase_count<FruitType::ORANGE>();
    fruit.print();

    return 0;
}

which removes the branch at compile time and fruit.increase_count<FruitType::APPLE> effectively calls increase_count_apple and fruit.increase_count<FruitType::ORANGE>() calls increase_count_orange.