C++ 中placement new 操作符使用方法

C++ 中placement new 操作符使用方法

placement new操作符能够在分配内存时指定内存位置。下面的程序使用了placement new操作符和常规new操作符给对象分配内存。

// placenew.cpp -- new, placement new, no delete
#include <iostream>
#include <string>
#include <new>

using namespace std;
const int BUF = 512;

class JustTesting
{
private:
  string words;
  int number;
public:
  JustTesting(const string &s = "Just Testing", int n = 0)
  {
    words = s; number = n; cout << words << " constructed\n";
  }
  ~JustTesting() { cout << words << " destroyed\n"; }
  void Show() const { cout << words << ", " << number << endl; }
};

int main(void)
{
  char *buffer = new char [BUF];  // get a block of memory
  JustTesting *pc1, *pc2;

  pc1 = new (buffer)JustTesting;  // place object in buffer
  pc2 = new JustTesting("heap1", 20);  // place object on heap

  cout << "Memory block address:\n" << "buffer: "
    << (void *)buffer << "  heap: " << pc2 << endl;
  cout << "Memory contents: \n";
  cout << pc1 << ": ";
  pc1->Show();
  cout << pc2 << ": ";
  pc2->Show();

  JustTesting *pc3, *pc4;
  pc3 = new (buffer) JustTesting("bad Idea", 6);
  pc4 = new JustTesting("Heap2", 10);

  cout << "Memory contents: \n";
  cout << pc3 << ": ";
  pc3->Show();
  cout << pc4 << ": ";
  pc4->Show();

  delete pc2;  // free heap1
  delete pc4;  // free heap2
  delete [] buffer;  // free buffer
  cout << "Done\n";

  return 0;
}

执行结果：

[root@localhost 桌面]# ./new 
Just Testing constructed
heap1 constructed
Memory block address:
buffer: 0x936a008  heap: 0x936a248
Memory contents: 
0x936a008: Just Testing, 0
0x936a248: heap1, 20
bad Idea constructed
Heap2 constructed
Memory contents: 
0x936a008: bad Idea, 6
0x936a290: Heap2, 10
heap1 destroyed
Heap2 destroyed
Done

上面的程序使用placement new操作时存在两个问题。首先，在创建第二个对象时，placement new操作符使用一个新对象来覆盖用于第一个对象的内存单元。显然，如果类动态地为其成员分配内存，这将引发问题。

其次，将delete用于pc2和pc4时，将自动调用为pc2和pc4指向的对象调用析构函数；然而，将delete[]用于buffer时，不会为使用布局new操作符创建的对象调用析构函数。

为确定两个单元不重叠，可以这样做：

pc1 = new (buffer) JustTesting;
pc3 = new (buffer + sizeof(JustTesting)) JustTesting("Better Idea", 6);

其中指针pc3相对于pc1的偏移量为JustTesting对象的大小

第二个教训是，如果使用placement new操作符来为对象分配内存，必须确保其析构函数被调用，但如何确保呢？

例如，在堆中创建的对象，可以这样做：

delete pc2;

然而，对于使用placement new操作符创建的对象，不能像下面一样调用delete

delete pc1; // NO!!!

原因在于delete可与常规new操作符配合使用，但不能与placement new操作符配合使用。

那么我们要显示调用析构函数，必须指定要销毁的对象：

pc3->~JustTesting(); // destroy object pointed to by pc3

int main(void)
{
  char *buffer = new char[BUF];  // get a block of memory
  JustTesting *pc1, *pc2;

  pc1 = new (buffer) JustTesting;  // place object in buffer
  pc2 = new JustTesting("Heap1", 20);  // place object on heap

  cout << "Memory block addresses: /n" << "buffer: "
    << (void *)buffer << "  heap: " << pc2 << endl;
  cout << "Memory contents: ";
  cout << pc1 << ": ";
  pc1->Show();
  cout << pc2 << ": ";
  pc2->Show();

  JustTesting *pc3, *pc4;
  // fix placement new location
  pc3 = new (buffer + sizeof(JustTesting)) JustTesting("better Idea", 6);
  pc4 = new JustTesting("Heap2", 10);

  cout << "Memory contents: ";
  cout << pc3 << ": ";
  pc3->Show();
  cout << pc4 << ": ";
  pc4->Show();

  delete pc2;    // free heap1
  delete pc4;    // free heap2
  // explicitly destroy placement new object
  pc3->~JustTesting();  // destroy object pointed to by pc3
  pc1->~JustTesting();  // destroy object pointed to by pc1
  delete []buffer;  // free buffer
  cout << "Done/n";

  return 0;
}

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