问题的主要部分是使用基于策略的设计和可变参数模板的CRTP。从策略无法访问主/派生类的受保护或私有成员。由于使用可变参数模板，我不能将策略声明为朋友。
问题是，如何将所有策略类设置为派生类的朋友。
鉴于此 CRTP 解决方案，什么是支持多继承级别并解决了没有虚拟继承的菱形问题。
// Derived - We would like to obtain access to this type of instance
// BaseDerived - Helper type to avoid the diamond problem without virtual inheritance
template<typename Derived, template<typename> class BaseDerived>
class Crtp {
protected:
[[nodiscard]] constexpr Derived & underlying() noexcept
{
return static_cast<Derived &>(*this);
}
[[nodiscard]] constexpr Derived const & underlying() const noexcept
{
return static_cast<Derived const &>(*this);
}
};
// Helper struct to achive multiple inheritance
struct NoDerivedClassTag;
template<template<typename> class Derived, typename Substitute, template<typename> class Base>
using DerivedCrtpBase = Base<std::conditional_t<std::is_same_v<Substitute, NoDerivedClassTag>, Derived<NoDerivedClassTag>, Substitute>>;
template<template<typename> class Interface, typename Object>
using is_crtp_interface_of = std::enable_if_t<
std::is_same_v<Interface<NoDerivedClassTag>, Object> || std::is_base_of_v<Interface<typename Object::exact_type>, Object>>;
在带有可变参数模板的基于策略的设计中使用此 CRTP 解决方案
template<template<typename> class... Functionality>
class FinalDerived
: public Functionality<FinalDerived<Functionality...>>...
{
public:
constexpr int get() const
{
return protected_variable_;
}
// Remove to check the problem
//protected:
int protected_variable_ {-1};
};
目标是像这样使用策略中的受保护变量
template<typename Derived>
struct Increment
: Crtp<Derived, Increment>
{
void increment(int an_value)
{
this->underlying().protected_variable_ += an_value;
}
};
template<typename Derived>
struct Decrement
: Crtp<Derived, Decrement>
{
void decrement(int an_value)
{
this->underlying().protected_variable_ -= an_value;
}
};
使用示例
constexpr int number {7};
int main(void){
FinalDerived<Increment, Decrement> derived;
std::cout << "start: " << derived.get() << "\n";
derived.increment(number);
std::cout << "incremented: " << derived.get() << "\n";
derived.decrement(number);
std::cout << "decremented: " << derived.get() << "\n";
}