C++ 中placement new 操作符使用方法分享

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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 << " constructedn";    }    ~JustTesting() { cout << words << " destroyedn"; }    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 << "Donen";      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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