C++如何在以GraphTypes为模板参数的类中访问其可变参数

错误原因分析 #

1. C3520错误：模板参数包是一组类型的集合，不能直接赋值给单个using类型别名，必须展开或者用模板容器（比如std::tuple）包裹。
2. C3543错误：你之前定义的ContextType不是参数包，后续代码尝试展开它自然会报错。

解决方案 #

通过对Edge类做模板偏特化，直接从GraphTypes的模板参数中提取出Context参数包，不需要在GraphTypes内部做复杂的类型定义，就可以直接使用参数包，同时兼容无Context的场景。

修改后完整代码 #

#include <initializer_list>

// 首先修改GraphTypes，添加可变参数包
template<class GraphT, class NodeT, class EdgeT, class... ContextT>
class GraphTypes {
public:
    using GraphType = GraphT;
    using NodeType = NodeT;
    using EdgeType = EdgeT;
};

// 前置声明Edge
template<class GraphTypes>
class Edge;

// Edge偏特化：直接提取GraphTypes的所有模板参数，包括Context包
template<template<typename, typename, typename, typename...> class GraphTypesT,
         class GraphT, class NodeT, class EdgeT, class... ContextT>
class Edge<GraphTypesT<GraphT, NodeT, EdgeT, ContextT...>> {
private:
    using GraphType = GraphT;
    using NodeType = NodeT;
    using EdgeType = EdgeT;

public:
    NodeType &GetSource();
    void SetSource(NodeType &Source, ContextT&... Context);

protected:
    NodeType *m_pSource = nullptr;
};

// GetSource实现
template<template<typename, typename, typename, typename...> class GraphTypesT,
         class GraphT, class NodeT, class EdgeT, class... ContextT>
inline NodeType &Edge<GraphTypesT<GraphT, NodeT, EdgeT, ContextT...>>::GetSource() {
    return *m_pSource;
}

// SetSource实现
template<template<typename, typename, typename, typename...> class GraphTypesT,
         class GraphT, class NodeT, class EdgeT, class... ContextT>
void Edge<GraphTypesT<GraphT, NodeT, EdgeT, ContextT...>>::SetSource(NodeType &Source, ContextT&... Context) {
    if(&Source == m_pSource)
        return;
      
    // 空参数包时展开也不会报错，C++11及以上都支持
    std::initializer_list<bool>{ (Context.PreSetSource(*this), true)... };

    m_pSource = &Source;

    std::initializer_list<bool>{ (Context.PostSetSource(*this), true)... };
}

使用示例 #

无上下文场景 #

class MyGraph; class MyNode; class MyEdge;
// 不需要Context时直接传三个参数即可
using MyGraphTypes = GraphTypes<MyGraph, MyNode, MyEdge>;
class MyEdge : public Edge<MyGraphTypes> {};

// 调用时不需要传Context
MyNode n;
MyEdge e;
e.SetSource(n);

带上下文场景 #

struct Context1 {
    void PreSetSource(MyEdge& e) {}
    void PostSetSource(MyEdge& e) {}
};
struct Context2 {
    void PreSetSource(MyEdge& e) {}
    void PostSetSource(MyEdge& e) {}
};

// 带两个上下文的定义
using MyGraphTypesWithCtx = GraphTypes<MyGraph, MyNode, MyEdge, Context1, Context2>;
class MyEdgeWithCtx : public Edge<MyGraphTypesWithCtx> {};

// 调用时传入对应的Context对象
Context1 c1;
Context2 c2;
MyNode n;
MyEdgeWithCtx e;
e.SetSource(n, c1, c2);

内容的提问来源于stack exchange，提问作者Magnar Myrtveit