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。
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// 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:
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[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
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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:
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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
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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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原文链接:http://www.cnblogs.com/wuchanming/p/4340268.html