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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 << " 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;
}
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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</div>
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);
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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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