[C++] placement new

개요

• 지정한 메모리에 객체를 초기화
• 메모리 할당과 초기화를 분리하기 위한 기법
• 메모리 할당 시점에 타입을 정할 수 없거나 초기화 시점을 지정하고 싶은 경우에 사용
  • pool(connection, momory, …), 다형성 등

주의사항

• placement new 사용 시 delete 대신 소멸자를 직접 호출하고 메모리 해제는 할당한 곳에서 할당 방법에 따른 적절한 해제를 해야함
  • delete의 경우 객체 소멸과 operator delete를 통한 메모리 해제 수행
  • placement new에서 할당한 메모리가 아니기 때문에 객체 소멸만 책임을 지는 것이 논리적

예제

• 코드

   #include <cstdlib>
   #include <iostream>
   #include <string>
      
   using namespace std;
      
   class Base {
   	protected:
   		int i;
      
   	public:
   		Base(int i) : i(i) { cout << "Base()" << endl; }
   		virtual ~Base() { cout << "~Base()" << endl; }
      
   		virtual void Print() = 0;
   };
      
   class Derived_1 : public Base {
   	private:
   		char c;
      
   	public:
   		Derived_1(int i, char c) : Base(i), c(c) {
   			cout << "Derived_1()" << endl;
   		}
   		virtual ~Derived_1() { cout << "~Derived_1()" << endl; }
      
   		virtual void Print() override {
   			cout << this->i << ", " << this->c << endl;
   		}
      
   		void Func1() { cout << "Func1()" << endl; };
   };
      
   class Derived_2 : public Base {
   	private:
   		string s;
      
   	public:
   		Derived_2(int i, string s) : Base(i), s(s) {
   			cout << "Derived_2()" << endl;
   		}
   		virtual ~Derived_2() { cout << "~Derived_2()" << endl; }
      
   		virtual void Print() override {
   			cout << this->i << ", " << this->s << endl;
   		}
      
   		void Func2() { cout << "Func2()" << endl; };
   };
      
   void malloc_test() {
   	cout << "--- " << __func__ << " start ---" << endl;
      
   	Base *base = static_cast<Base *>(malloc(sizeof(Base) * 1));
      
   	cout << "~~~" << endl;
      
   	Derived_1 *derived = new (base) Derived_1(1, 'a');
      
   	base->Print();
   	derived->Print();
      
   	static_cast<Derived_1 *>(base)->Func1();
      
   	base->~Base();
      
   	free(base);
      
   	cout << "--- " << __func__ << " end ---" << endl;
   }
      
   void calloc_test() {
   	cout << "--- " << __func__ << " start ---" << endl;
      
   	Base *base = static_cast<Base *>(calloc(1, sizeof(Base)));
      
   	cout << "~~~" << endl;
      
   	Derived_1 *derived = new (base) Derived_1(1, 'b');
      
   	base->Print();
   	derived->Print();
      
   	static_cast<Derived_1 *>(base)->Func1();
      
   	base->~Base();
      
   	free(base);
      
   	cout << "--- " << __func__ << " end ---" << endl;
   }
      
   void operator_new_test_1() {
   	cout << "--- " << __func__ << " start ---" << endl;
      
   	Base *base = static_cast<Base *>(operator new(sizeof(Base)));
      
   	cout << "~~~" << endl;
      
   	Derived_2 *derived = new (base) Derived_2(1, "aaa");
      
   	base->Print();
   	derived->Print();
      
   	static_cast<Derived_2 *>(base)->Func2();
      
   	base->~Base();
      
   	operator delete(base);
      
   	cout << "--- " << __func__ << " end ---" << endl;
   }
      
   void operator_new_test_2() {
   	cout << "--- " << __func__ << " start ---" << endl;
      
   	const int size = 3;
      
   	unsigned char pool[sizeof(Derived_2) * size];
      
   	for (int i = 0; i < size; ++i) {
   		new (pool + (sizeof(Derived_2) * i)) Derived_2(i, "a");
   	}
      
   	for (int i = 0; i < size; ++i) {
   		reinterpret_cast<Derived_2 *>(pool + sizeof(Derived_2) * i)->Print();
   	}
      
   	for (int i = 0; i < size; ++i) {
   		reinterpret_cast<Derived_2 *>(pool + sizeof(Derived_2) * i)
   			->~Derived_2();
   	}
      
   	cout << "--- " << __func__ << " end ---" << endl;
   }
      
   int main() {
   	malloc_test();
      
   	cout << endl;
      
   	calloc_test();
      
   	cout << endl;
      
   	operator_new_test_1();
      
   	cout << endl;
      
   	operator_new_test_2();
      
   	return 0;
   }
  

• 실행 결과

   --- malloc_test start ---
   ~~~
   Base()
   Derived_1()
   1, a
   1, a
   Func1()
   ~Derived_1()
   ~Base()
   --- malloc_test end ---
      
   --- calloc_test start ---
   ~~~
   Base()
   Derived_1()
   1, b
   1, b
   Func1()
   ~Derived_1()
   ~Base()
   --- calloc_test end ---
      
   --- operator_new_test_1 start ---
   ~~~
   Base()
   Derived_2()
   1, aaa
   1, aaa
   Func2()
   ~Derived_2()
   ~Base()
   --- operator_new_test_1 end ---
      
   --- operator_new_test_2 start ---
   Base()
   Derived_2()
   Base()
   Derived_2()
   Base()
   Derived_2()
   0, a
   1, a
   2, a
   ~Derived_2()
   ~Base()
   ~Derived_2()
   ~Base()
   ~Derived_2()
   ~Base()
   --- operator_new_test_2 end ---