问题描述
我使用可变参数包来进行基于策略的类设计。
I am using variadic parameter packs for policy based class design.
template <APITypes APIType, class... Policies>
class IShader : public Policies... {
};
策略在调用时定义，如果未指定则使用默认值。问题出现在我需要添加另一个变量参数包时：
Policies are defined when called or with defaults if none are specified. The problem comes when I need to add another variadic parameter pack:
template <AttributeType... Attributes, APITypes APIType, class... Policies>
class IShader : public Policies... {
};
这会导致错误模板参数包必须是最后一个模板参数。我计划使用属性包更改至少一个策略的行为。但我无法弄清楚如何在一个模板类中获取两个可变参数包。
This results in the error "Template parameter pack must be the last template parameter". I am planning to use the attribute pack to change the behaviour of at least one of the policies. But I can't work out how to get two variadic parameter packs in one template class.
推荐答案
或者属性列表的某种包装器。
In the discussion comments you expressed a willingness to consider some kind of indirection, or "a wrapper of some kind for the attribute list".
轻量级 std :: tuple 的包装器与专业化可能在这里工作：
A lightweight std::tuple-based wrapper, together with specialization, might work here:
template <typename attribute_tuple, APITypes APIType,
typename policy_tuple> class IShader;
template <AttributeType... Attributes, APITypes APIType,
class... Policies>
class IShader<std::tuple<Attributes...>, APIType,
std::tuple<Policies...>> : public Policies... {
// ...
};
这里的目标是使用以下行的模板实例：
The goal here is to use a template instance along the lines of:
IShared<std::tuple<Attribute1, Attribute2>, APITypeFoo,
std::tuple<Policy1, Policy2>> ishared_instance;
然后交叉你的手指，这将是专门的模板声明，可供模板专业化单独使用。
And cross your fingers that this is going to match the specialized template declaration, at which point both parameter packs are available for the template specialization to use, individually.
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