An example of builder pattern in c++

builder_pattern.cpp

#include <stdio.h>
#include <memory>
class BBuilder;

class A {
 public:
  A() {}
  virtual ~A() { fprintf(stderr, "A done.\n"); }

  int getA() const { return _a; }

  virtual void print() const { fprintf(stderr, "_a=%d\n", _a); }

 private:
  int _a = 101;
};

class B : public A {
  friend BBuilder;

 public:
  virtual ~B() { fprintf(stderr, "B done.\n"); }

  int getParam() const { return _param; }
  int getAnotherParam() const { return _another_param; }
  void print() const override {
    fprintf(stderr, "_param=%d\n", _param);
    fprintf(stderr, "_another_param=%d\n", _another_param);
  }

 private:
  B() : A{} {}
  int _param = 20;
  int _another_param = 20;
};

class BBuilder {
 public:
  BBuilder() { _b.reset(new B); }
  virtual ~BBuilder() { fprintf(stderr, "Builder done.\n"); }

  BBuilder& withParam(int param) {
    if (_b) {
      _b->_param = param;
    }
    return *this;
  }

  BBuilder& withAnotherParam(int param) {
    if (_b) {
      _b->_another_param = param;
    }
    return *this;
  }

  std::unique_ptr<B> build() { return std::move(_b); }

 private:
  std::unique_ptr<B> _b = nullptr;
};

int main(int argc, char const* argv[]) {
  std::unique_ptr<A> a =
      BBuilder().withParam(100).withAnotherParam(200).build();
  fprintf(stderr, "Builder should be dead now.\n");
  a->print();
  return 0;
}