如何在可变参数类模板中获取类型的索引？ [英] How can I get the index of a type in a variadic class template?

问题描述

我有一个可变参数引擎模板类：

I have a variadic Engine template class:

template <typename ... Components> class Engine;

我想在编译时为每个组件分配一个编号，该编号等于它们的顺序。进行以下调用时将返回此值：

I'd like to assign a number to each component at compile time which is equivalent to their ordering. This would be returned when making the following call:

template <typename Component> int ordinal();

例如，如果：

Engine<PositionComponent, PhysicsComponent, InputComponent> engine;

已声明，调用：

engine.ordinal<PhysicsComponent>();

将返回1，使用InputComponent而不是PhysicsComponent的类似调用将返回2。

would return 1 and a similar call with InputComponent instead of PhysicsComponent would return 2.

有可能吗，如果可以，人们会怎么做？

Is it possible, and if yes, how would one go about it?

推荐答案

因此，您想在<$中找到 Component 的索引c $ c>组件... ？

So you want to find the index of Component in Components...?

template <typename... >
struct index;

// found it
template <typename T, typename... R>
struct index<T, T, R...>
: std::integral_constant<size_t, 0>
{ };

// still looking
template <typename T, typename F, typename... R>
struct index<T, F, R...>
: std::integral_constant<size_t, 1 + index<T,R...>::value>
{ };

用法：

template <typename Component> 
size_t ordinal() { return index<Component, Components...>::value; }

按构造，试图获取普通不在 Components ... 中的 Component 将是编译错误。似乎合适。

As constructed, trying to get the ordinal of a Component not in Components... will be a compile error. Which seems appropriate.

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