new和delete时究竟发生了什么?
来源:互联网 发布:淘宝送货 编辑:程序博客网 时间:2024/04/28 14:35
在C++中,当用户调用new和delete时,实际上分别发生了如下事情:
new:
1. 调用::operator new(相当于malloc), 分配一块内存;
2. 检查第一步内存分配的结果,如果未被正常分配(比如系统资源耗尽时),抛出异常,直接返回,接下来的步骤不会执行;
3. 调用构造函数.
delete:
1. 检查指定要删除的内存,如果已经为空,直接返回,接下来的步骤不会执行;
2. 调用析构函数;
3. 调用::operator delete(相当于free),释放内存.
下面我们通过查看测试代码的相应汇编代码来证实上述观点.
测试代码:
#include "stdafx.h"
#include <iostream>
using namespace std;
class MyClass
...{
public:
MyClass(const char*) ...{}
~MyClass() ...{}
operator char() ...{return 'a';}
protected:
private:
char* m_data;
};
int _tmain(int argc, _TCHAR* argv[])
...{
MyClass* my = new MyClass("Hello!");
delete my;
char memory[sizeof(MyClass)];
void* p = memory;
MyClass* second = new (p) MyClass("World!");
second->~MyClass();
return 0;
}
#include <iostream>
using namespace std;
class MyClass
...{
public:
MyClass(const char*) ...{}
~MyClass() ...{}
operator char() ...{return 'a';}
protected:
private:
char* m_data;
};
int _tmain(int argc, _TCHAR* argv[])
...{
MyClass* my = new MyClass("Hello!");
delete my;
char memory[sizeof(MyClass)];
void* p = memory;
MyClass* second = new (p) MyClass("World!");
second->~MyClass();
return 0;
}
相应的汇编代码如下:
int _tmain(int argc, _TCHAR* argv[])
...{
004136E0 push ebp
……
MyClass* my = new MyClass("Hello!");
0041371D push 4
0041371F call operator new (411190h) // 步骤1
00413724 add esp,4
00413727 mov dword ptr [ebp-134h],eax
0041372D mov dword ptr [ebp-4],0
00413734 cmp dword ptr [ebp-134h],0
0041373B je wmain+75h (413755h) // 步骤2
0041373D push offset string "Hello!" (41676Ch)
00413742 mov ecx,dword ptr [ebp-134h]
00413748 call MyClass::MyClass (41116Dh) // 步骤3
0041374D mov dword ptr [ebp-148h],eax
00413753 jmp wmain+7Fh (41375Fh)
00413755 mov dword ptr [ebp-148h],0
0041375F mov eax,dword ptr [ebp-148h]
00413765 mov dword ptr [ebp-140h],eax
0041376B mov dword ptr [ebp-4],0FFFFFFFFh
00413772 mov ecx,dword ptr [ebp-140h]
00413778 mov dword ptr [ebp-14h],ecx
delete my;
0041377B mov eax,dword ptr [ebp-14h]
0041377E mov dword ptr [ebp-11Ch],eax
00413784 mov ecx,dword ptr [ebp-11Ch]
0041378A mov dword ptr [ebp-128h],ecx
00413790 cmp dword ptr [ebp-128h],0
00413797 je wmain+0CEh (4137AEh) // 步骤1
00413799 push 1
0041379B mov ecx,dword ptr [ebp-128h]
004137A1 call MyClass::`scalar deleting destructor' (41100Ah) // 步骤2和步骤3
004137A6 mov dword ptr [ebp-148h],eax
004137AC jmp wmain+0D8h (4137B8h)
004137AE mov dword ptr [ebp-148h],0
char memory[sizeof(MyClass)];
void* p = memory;
004137B8 lea eax,[ebp-20h]
004137BB mov dword ptr [ebp-2Ch],eax
MyClass* second = new (p) MyClass("World!");
004137BE mov eax,dword ptr [ebp-2Ch]
004137C1 push eax
004137C2 push 4
004137C4 call operator new (4111E0h) // 步骤1
004137C9 add esp,8
004137CC mov dword ptr [ebp-104h],eax
004137D2 mov dword ptr [ebp-4],1
004137D9 cmp dword ptr [ebp-104h],0
004137E0 je wmain+11Ah (4137FAh) // 步骤2
004137E2 push offset string "World!" (4166FCh)
004137E7 mov ecx,dword ptr [ebp-104h]
004137ED call MyClass::MyClass (41116Dh) // 步骤3
004137F2 mov dword ptr [ebp-148h],eax
004137F8 jmp wmain+124h (413804h)
004137FA mov dword ptr [ebp-148h],0
00413804 mov ecx,dword ptr [ebp-148h]
0041380A mov dword ptr [ebp-110h],ecx
00413810 mov dword ptr [ebp-4],0FFFFFFFFh
00413817 mov edx,dword ptr [ebp-110h]
0041381D mov dword ptr [ebp-38h],edx
second->~MyClass();
00413820 push 0
00413822 mov ecx,dword ptr [ebp-38h]
00413825 call MyClass::`scalar deleting destructor' (41100Ah)
return 0;
0041382A xor eax,eax
}
0041382C push edx
……
...{
004136E0 push ebp
……
MyClass* my = new MyClass("Hello!");
0041371D push 4
0041371F call operator new (411190h) // 步骤1
00413724 add esp,4
00413727 mov dword ptr [ebp-134h],eax
0041372D mov dword ptr [ebp-4],0
00413734 cmp dword ptr [ebp-134h],0
0041373B je wmain+75h (413755h) // 步骤2
0041373D push offset string "Hello!" (41676Ch)
00413742 mov ecx,dword ptr [ebp-134h]
00413748 call MyClass::MyClass (41116Dh) // 步骤3
0041374D mov dword ptr [ebp-148h],eax
00413753 jmp wmain+7Fh (41375Fh)
00413755 mov dword ptr [ebp-148h],0
0041375F mov eax,dword ptr [ebp-148h]
00413765 mov dword ptr [ebp-140h],eax
0041376B mov dword ptr [ebp-4],0FFFFFFFFh
00413772 mov ecx,dword ptr [ebp-140h]
00413778 mov dword ptr [ebp-14h],ecx
delete my;
0041377B mov eax,dword ptr [ebp-14h]
0041377E mov dword ptr [ebp-11Ch],eax
00413784 mov ecx,dword ptr [ebp-11Ch]
0041378A mov dword ptr [ebp-128h],ecx
00413790 cmp dword ptr [ebp-128h],0
00413797 je wmain+0CEh (4137AEh) // 步骤1
00413799 push 1
0041379B mov ecx,dword ptr [ebp-128h]
004137A1 call MyClass::`scalar deleting destructor' (41100Ah) // 步骤2和步骤3
004137A6 mov dword ptr [ebp-148h],eax
004137AC jmp wmain+0D8h (4137B8h)
004137AE mov dword ptr [ebp-148h],0
char memory[sizeof(MyClass)];
void* p = memory;
004137B8 lea eax,[ebp-20h]
004137BB mov dword ptr [ebp-2Ch],eax
MyClass* second = new (p) MyClass("World!");
004137BE mov eax,dword ptr [ebp-2Ch]
004137C1 push eax
004137C2 push 4
004137C4 call operator new (4111E0h) // 步骤1
004137C9 add esp,8
004137CC mov dword ptr [ebp-104h],eax
004137D2 mov dword ptr [ebp-4],1
004137D9 cmp dword ptr [ebp-104h],0
004137E0 je wmain+11Ah (4137FAh) // 步骤2
004137E2 push offset string "World!" (4166FCh)
004137E7 mov ecx,dword ptr [ebp-104h]
004137ED call MyClass::MyClass (41116Dh) // 步骤3
004137F2 mov dword ptr [ebp-148h],eax
004137F8 jmp wmain+124h (413804h)
004137FA mov dword ptr [ebp-148h],0
00413804 mov ecx,dword ptr [ebp-148h]
0041380A mov dword ptr [ebp-110h],ecx
00413810 mov dword ptr [ebp-4],0FFFFFFFFh
00413817 mov edx,dword ptr [ebp-110h]
0041381D mov dword ptr [ebp-38h],edx
second->~MyClass();
00413820 push 0
00413822 mov ecx,dword ptr [ebp-38h]
00413825 call MyClass::`scalar deleting destructor' (41100Ah)
return 0;
0041382A xor eax,eax
}
0041382C push edx
……
这里需要特别注意的是:
1. 定位创建(placement new)的方法, 即new(p) MyClass("Hello"),不管p是否被正确创建,定位创建的过程与一般创建的过程是一样的,即也经历3个主要步骤.
2. 汇编代码中所说的scalar deleting destructor是编译器自动生成的, 相当于先执行析构函数再执行delete操作,相关信息可参考http://www.thescripts.com/forum/thread263644.html
- new和delete时究竟发生了什么?
- 调用new/delete时究竟发生了什么----Effective C++学习笔记
- new/delete究竟做了些什么
- C++中new和delete之后发生了什么
- 《访问网页时究竟发生了些什么》-知识点
- new之后发生了什么?
- Javascript new发生了什么?
- js new 发生了什么
- main函数之前究竟发生了什么?
- main函数之前究竟发生了什么?
- javaScript使用new关键字时发生了什么?
- new HashMap 发生了什么事情?
- JS new对象内部发生了什么
- 输入URL按下回车后究竟发生了什么
- gcc -static hello.c 背后究竟发生了什么
- 人人网市值缩水近80%,究竟发生了什么?
- suspend和resume究竟做了什么?
- java笔记--Day07--new一个对象时,内存中发生了什么?
- 在 ASP/ASP.NET 中处理客户端数字证书
- .NET后台智能传输服务实现自动更新
- 开始学习 SCJP !
- WindowXP操作系统快捷键一览表
- 保存远程图片到本地文件夹!
- new和delete时究竟发生了什么?
- 编写通用的ASP防SQL注入攻击程序
- ASP.net防SQL注入
- C语言编程 代码优化 经验谈
- FSO读取一个文件相关信息的范例
- 成员方法 参数 调用 static final
- 强硬+铁腕
- Linux新手入门教程
- PHP5.x时差问题