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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 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;
}
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