C++ 中placement new 操作符使用方法
placement new操作符能夠在分配內存時指定內存位置。下面的程序使用了placement new操作符和常規new操作符給對象分配內存。
// 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;}執行結果：
[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操作時存在兩個問題。首先，在創建第二個對象時，placement new操作符使用一個新對象來覆蓋用于第一個對象的內存單元。顯然，如果類動態地為其成員分配內存，這將引發問題。
其次，將delete用于pc2和pc4時，將自動調用為pc2和pc4指向的對象調用析構函數；然而，將delete[]用于buffer時，不會為使用布局new操作符創建的對象調用析構函數。
為確定兩個單元不重疊，可以這樣做：
pc1 = new (buffer) JustTesting;pc3 = new (buffer + sizeof(JustTesting)) JustTesting("Better Idea", 6);其中指針pc3相對于pc1的偏移量為JustTesting對象的大小
第二個教訓是，如果使用placement new操作符來為對象分配內存，必須確保其析構函數被調用，但如何確保呢？
例如，在堆中創建的對象，可以這樣做：
delete pc2;
然而，對于使用placement new操作符創建的對象，不能像下面一樣調用delete
delete pc1; // NO!!!
原因在于delete可與常規new操作符配合使用，但不能與placement new操作符配合使用。
那么我們要顯示調用析構函數，必須指定要銷毀的對象：
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;}感謝閱讀，希望能幫助到大家，謝謝大家對本站的支持！
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