如何在可变参数派生类中执行 crtp 基类的所有函数？

Question

我有一个CRTP派生类，它是它可以继承的所有 CRTP 基类的可变模板。我想在派生类的方法（printAll 函数）中执行每个继承类的函数（在本例中为 print 函数）。我怎样才能做到这一点？

// Base Class 1
template<typename Derived>
struct Mult
{
  void print()
  {
    int a = (static_cast<Derived const&>(*this)).m_a;
    int b = (static_cast<Derived const&>(*this)).m_b;
    std::cout << "a * b: " << a * b << "\n";
  }
};

// Base Class 2
template<typename Derived>
struct Add
{
  void print()
  {
    int a = (static_cast<Derived const&>(*this)).m_a;
    int b = (static_cast<Derived const&>(*this)).m_b;
    std::cout << "a + b: " << a + b << "\n";
  }
};

template<template<typename> typename... Bases>
struct Derived : public Bases<Derived<Bases...>>...
{
  int m_a, m_b;
  Derived(int a, int b) : m_a(a), m_b(b) {}
  void printAll()
  {
    // Should execute the print method of all the derived classes
    this->print();
  }
};


int main()
{
  Derived<Mult, Add> d(2, 3);
  // should print:
  // a + b: 5
  // a * b: 6
  d.printAll();
}

Answer 1

您可以使用折叠表达式，它是 C++17 中的新语言功能之一：

void printAll()
{
    (Bases<Derived>::print(), ...);
}