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用汇编的眼光看C++(之类静态变量、静态函数)

2011-10-06

【 声明:版权所有,欢迎转载,请勿用于商业用途。 联系信箱:feixiaoxing @163.com】 看过设计模式的朋友都知道,我们在设计单件模式的时候离不开类的静态函数。和类的成员变量不同,类的静态变量属于全部类...

【 声明:版权所有,欢迎转载,请勿用于商业用途。 联系信箱:feixiaoxing @163.com】

看过设计模式的朋友都知道,我们在设计单件模式的时候离不开类的静态函数。和类的成员变量不同,类的静态变量属于全部类对象数据;同样和类的成员函数不同,类的静态函数属于全部类函数共有。这句话读来想来有一些拗口,不过没有关系,我们可以通过一段代码来说明问题。

(1)静态变量

老规矩,我们首先对类进行初步定义,如下所示:

class employee

{

public:

employee() { }

~employee() {}

static int value;

};

class employee

{

public:

employee() { }

~employee() {}

static int value;

}; 那么,这里出现的value是不是所共有的,我们可以看看相关的函数代码即可:

67: int employee::value = 0;

68:

69: void process()

70: {

00401240 push ebp

00401241 mov ebp,esp

00401243 push 0FFh

00401245 push offset __ehhandler$?process@@YAXXZ (0041f469)

0040124A mov eax,fs:[00000000]

00401250 push eax

00401251 mov dword ptr fs:[0],esp

00401258 sub esp,48h

0040125B push ebx

0040125C push esi

0040125D push edi

0040125E lea edi,[ebp-54h]

00401261 mov ecx,12h

00401266 mov eax,0CCCCCCCCh

0040126B rep stos dword ptr [edi]

71: employee m;

0040126D lea ecx,[ebp-10h]

00401270 call @ILT+35(employee::employee) (00401028)

00401275 mov dword ptr [ebp-4],0

72: employee n;

0040127C lea ecx,[ebp-14h]

0040127F call @ILT+35(employee::employee) (00401028)

73:

74: m.value = 10;

00401284 mov dword ptr [employee::value (00438494)],0Ah

75: n.value = 100;

0040128E mov dword ptr [employee::value (00438494)],64h

76: }

00401298 lea ecx,[ebp-14h]

0040129B call @ILT+0(employee::~employee) (00401005)

004012A0 mov dword ptr [ebp-4],0FFFFFFFFh

004012A7 lea ecx,[ebp-10h]

004012AA call @ILT+0(employee::~employee) (00401005)

004012AF mov ecx,dword ptr [ebp-0Ch]

004012B2 mov dword ptr fs:[0],ecx

004012B9 pop edi

004012BA pop esi

004012BB pop ebx

004012BC add esp,54h

004012BF cmp ebp,esp

004012C1 call __chkesp (004086b0)

004012C6 mov esp,ebp

004012C8 pop ebp

004012C9 ret

67: int employee::value = 0;

68:

69: void process()

70: {

00401240 push ebp

00401241 mov ebp,esp

00401243 push 0FFh

00401245 push offset __ehhandler$?process@@YAXXZ (0041f469)

0040124A mov eax,fs:[00000000]

00401250 push eax

00401251 mov dword ptr fs:[0],esp

00401258 sub esp,48h

0040125B push ebx

0040125C push esi

0040125D push edi

0040125E lea edi,[ebp-54h]

00401261 mov ecx,12h

00401266 mov eax,0CCCCCCCCh

0040126B rep stos dword ptr [edi]

71: employee m;

0040126D lea ecx,[ebp-10h]

00401270 call @ILT+35(employee::employee) (00401028)

00401275 mov dword ptr [ebp-4],0

72: employee n;

0040127C lea ecx,[ebp-14h]

0040127F call @ILT+35(employee::employee) (00401028)

73:

74: m.value = 10;

00401284 mov dword ptr [employee::value (00438494)],0Ah

75: n.value = 100;

0040128E mov dword ptr [employee::value (00438494)],64h

76: }

00401298 lea ecx,[ebp-14h]

0040129B call @ILT+0(employee::~employee) (00401005)

004012A0 mov dword ptr [ebp-4],0FFFFFFFFh

004012A7 lea ecx,[ebp-10h]

004012AA call @ILT+0(employee::~employee) (00401005)

004012AF mov ecx,dword ptr [ebp-0Ch]

004012B2 mov dword ptr fs:[0],ecx

004012B9 pop edi

004012BA pop esi

004012BB pop ebx

004012BC add esp,54h

004012BF cmp ebp,esp

004012C1 call __chkesp (004086b0)

004012C6 mov esp,ebp

004012C8 pop ebp

004012C9 ret

我们直接看74行和75行。我们看到复制的对象地址都是惊人的一致,这说明实际上m和n所指的value实际上是同一个地址0x401005。不过,类的静态对象有一个要求,那就是对象的静态变量必须像全局变量一样进行初始化操作。

(2)静态函数

静态函数和静态变量一样,实际上就是类的全局函数。它之所以和普通的成员函数不一样,就是因为它不需要定义类的类型就能使用这个函数。根据上面的信息,我们可以重新定义一下这个类:

class employee

{

public:

employee() { }

~employee() {}

static void print() { printf("employee::print()!\n");};

};

class employee

{

public:

employee() { }

~employee() {}

static void print() { printf("employee::print()!\n");};

}; 那么类的静态函数是这样使用的呢?大家看看下面这样一个函数:

68: void process()

69: {

00401230 push ebp

00401231 mov ebp,esp

00401233 sub esp,40h

00401236 push ebx

00401237 push esi

00401238 push edi

00401239 lea edi,[ebp-40h]

0040123C mov ecx,10h

00401241 mov eax,0CCCCCCCCh

00401246 rep stos dword ptr [edi]

70: employee::print();

00401248 call @ILT+0(employee::print) (00401005)

71: }

0040124D pop edi

0040124E pop esi

0040124F pop ebx

00401250 add esp,40h

00401253 cmp ebp,esp

00401255 call __chkesp (00408620)

0040125A mov esp,ebp

0040125C pop ebp

0040125D ret

68: void process()

69: {

00401230 push ebp

00401231 mov ebp,esp

00401233 sub esp,40h

00401236 push ebx

00401237 push esi

00401238 push edi

00401239 lea edi,[ebp-40h]

0040123C mov ecx,10h

00401241 mov eax,0CCCCCCCCh

00401246 rep stos dword ptr [edi]

70: employee::print();

00401248 call @ILT+0(employee::print) (00401005)

71: }

0040124D pop edi

0040124E pop esi

0040124F pop ebx

00401250 add esp,40h

00401253 cmp ebp,esp

00401255 call __chkesp (00408620)

0040125A mov esp,ebp

0040125C pop ebp

0040125D ret

静态函数不需要对应的类对象,所以也就不需要this指针。这就是成员函数和静态函数的区别,仅此而已。

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