C++ 中placement new 操作符使用方法_C/C++

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。

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									// 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：

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									[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：

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									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

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									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;

									}