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 constructedheap1 constructedMemory block address:buffer: 0x936a008 heap: 0x936a248Memory contents: 0x936a008: Just Testing, 00x936a248: heap1, 20bad Idea constructedHeap2 constructedMemory contents: 0x936a008: bad Idea, 60x936a290: Heap2, 10heap1 destroyedHeap2 destroyedDone

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;}

感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！