用汇编的眼光看C++（之类静态变量、静态函数）
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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。
