c++ - 如何使用模板选择类(class)成员

Question

考虑下面的示例代码。

#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;
}

我希望 increase_count 函数有效地编译为 increase_count_apple 和 increase_count_orange。这是另一个类的简化示例，理论上，这种编译时优化在运行时对我有帮助。直观上，这感觉可以使用模板，但我无法获得令人满意的输出。

Answer 1

通过制作increase_count一个采用非类型模板参数的模板，您可以这样做

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;
}

它会在编译时删除分支并 fruit.increase_count<FruitType::APPLE>有效调用 increase_count_apple和fruit.increase_count<FruitType::ORANGE>()来电 increase_count_orange .