CODEWITHSUNDEEP
c++templatescrtp
Aleks96
Aleks96

Reputation: 35

How create few objects with CRTP?

I write small example CRTP pattern to better learn it and use it pattern in more complex code. I want to use CRTP, that base class have access to derived class. All Ok, but i can't create few objects for my base class. If i at first call constructor for both objects Base<Derived1> base1; Base<Derived2> base2;, and at second call function from each object base1.PrintDerived_FromA(); base2.PrintDerived_FromA();, i have result: 

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

but, i should have that: 

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

If i call function right after constructor, all Ok:

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

Result:

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

It turns out a new constructor call overrides the existing object, but why? It is possible to fix this? And i want to use only CRTP, no virtual functions.

#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;
}

Upvotes: 1

Views: 111

Answers (1)

user7860670
user7860670

Reputation: 37578

static_cast<Derived1*>(this) cast is invalid: this points at object of Base<Derived1> type not at Derived1. So dereferencing produced pointer causes Undefined Behavior. In order for CRTP to work you need to create objects of derived classes.

Upvotes: 1

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