英文:
How to select a class member using templates
问题
我想将 increase_count 函数有效地编译成 increase_count_apple 和 increase_count_orange。这是另一个类的简化示例，其中这种编译时优化在理论上可以帮助我在运行时。直观地感觉可以使用模板来实现，但我无法得出令人满意的结果。
请注意，这是您要求的代码部分的翻译。
英文:
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++;elseorange_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.
答案1
得分: 4
通过将 increase_count 设为一个接受非类型模板参数的模板，你可以做到：
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++;elseorange_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;}
这将在编译时移除分支，fruit.increase_count<FruitType::APPLE> 实际上调用 increase_count_apple，fruit.increase_count<FruitType::ORANGE>() 调用 increase_count_orange。
英文:
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++;elseorange_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.
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