c++ - 如何使用 CRTP 创建少量对象？

Question

我编写了一些小示例 CRTP 模式，以便更好地学习它并在更复杂的代码中使用它。我想使用 CRTP，该基类可以访问派生类。好的，但是我不能为我的基类创建一些对象。如果我首先为两个对象调用构造函数 Base<Derived1> base1; Base<Derived2> base2; ，然后从每个对象调用函数 base1.PrintDerived_FromA(); base2.PrintDerived_FromA(); ，我有结果:

Base constr work
Base constr work
b_: 0
b_: 25

但是，我应该有那个:

Base constr work
Base constr work
b_: 9
b_: 25

如果我在构造函数之后立即调用函数，一切正常:

Base<Derived1> base1;
base1.PrintDerived_FromA();
Base<Derived2> base2;
base2.PrintDerived_FromA();

结果:

Base constr work
b_: 9
Base constr work
b_: 25

事实证明，新的构造函数调用会覆盖现有对象，但为什么呢？有可能解决这个问题吗？而且我只想使用 CRTP，没有虚拟功能。

#include <iostream>

template <class T>
class Base {
 public:
  Base();
  void PrintDerived_FromA();
  void InitializeDerived();
};

class Derived1 : public Base<Derived1> {
 public:
  Derived1(int b);
  void PrintDerived();
  void SetDerived(int b);

 private:
  int b_;
};

class Derived2 : public Base<Derived2> {
 public:
  Derived2(int b);
  void PrintDerived();
  void SetDerived(int b);

 private:
  int b_;
};


template <typename T>
Base<T>::Base() {
  InitializeDerived();
  std::cout << "Base constr work" << std::endl;
}

template <>
void Base<Derived1>::InitializeDerived() {
  static_cast<Derived1*>(this)->SetDerived(9);
}

template <>
void Base<Derived2>::InitializeDerived() {
  static_cast<Derived2*>(this)->SetDerived(25);
}

template <typename T>
void Base<T>::PrintDerived_FromA() {
  static_cast<T*>(this)->PrintDerived();
}


Derived1::Derived1(int b) : b_(b), Base() {
  std::cout << "Derived1 constr work" << std::endl;
}

void Derived1::PrintDerived() {
  std::cout << "b_: " << b_ << std::endl;
}

void Derived1::SetDerived(int b) {
  b_ = b;
}


Derived2::Derived2(int b) : b_(b), Base() {
  std::cout << "Derived2 constr work" << std::endl;
}

void Derived2::PrintDerived() {
  std::cout << "b_: " << b_ << std::endl;
}

void Derived2::SetDerived(int b) {
  b_ = b;
}


int main() {
  Base<Derived1> base1;
  Base<Derived2> base2;

  base1.PrintDerived_FromA();
  base2.PrintDerived_FromA();

  return 0;
}

Answer 1

static_cast<Derived1*>(this)转换无效:this指向 Base<Derived1> 的对象类型不在 Derived1 .因此取消引用生成的指针会导致未定义的行为。为了让 CRTP 工作，您需要创建派生类的对象。