C++異常處理機制__3.編譯器何時生成棧回退代碼

 學習異常原理，先從幾個例子看起

編譯器什麼時候生成異常支持代碼(棧回退支持)
//沒有棧回退代碼 void cpp_exception_test1() { int *p = new int; CMyClass1 s1; } //有棧回退代碼 void cpp_exception_test2() { CMyClass1 s1; int *p = new int; } void func_no_throw() { } void func_has_throw() { throw 1; } //沒有棧回退代碼 void cpp_exception_test3() { CMyClass1 s1; func_no_throw(); } //有棧回退代碼 void cpp_exception_test4() { CMyClass1 s1; func_has_throw(); } //沒有棧回退代碼 void cpp_exception_test5() { throw 1; } //有棧回退代碼 void cpp_exception_test6() { try { } catch (...) { } }


觀察每個函數生成的彙編代碼 (VS2005, Release模式)

//沒有棧回退代碼 void cpp_exception_test1() { 00402380 push ebp 00402381 mov ebp,esp 00402383 sub esp,0Ch int *p = new int; 00402386 push 4 00402388 call operator new (4016DEh) 0040238D add esp,4 00402390 mov dword ptr [ebp-0Ch],eax 00402393 mov eax,dword ptr [ebp-0Ch] 00402396 mov dword ptr [p],eax CMyClass1 s1; 00402399 lea ecx,[s1] 0040239C call CMyClass1::CMyClass1 (4015A0h) } 004023A1 lea ecx,[s1] 004023A4 call CMyClass1::~CMyClass1 (4015D0h) 004023A9 mov esp,ebp 004023AB pop ebp 004023AC ret

可見函數1 沒有棧回退代碼。
雖然new 函數失敗時可能拋出異常，但編譯器判斷函數有異常時沒有局部類變量需要析構，
所以沒有棧回退代碼生成

//有棧回退代碼 void cpp_exception_test2() { 00402310 push ebp 00402311 mov ebp,esp 00402313 push 0FFFFFFFFh 00402315 push offset __ehhandler$?ThreadFunc1@@YGKPAX@Z+77h (402657h) 0040231A mov eax,dword ptr fs:[00000000h] 00402320 push eax 00402321 sub esp,0Ch 00402324 mov eax,dword ptr [___security_cookie (40501Ch)] 00402329 xor eax,ebp 0040232B push eax 0040232C lea eax,[ebp-0Ch] 0040232F mov dword ptr fs:[00000000h],eax CMyClass1 s1; 00402335 lea ecx,[ebp-11h] 00402338 call CMyClass1::CMyClass1 (4015A0h) 0040233D mov dword ptr [ebp-4],0 int *p = new int; 00402344 push 4 00402346 call operator new (4016DEh) 0040234B add esp,4 0040234E mov dword ptr [ebp-18h],eax 00402351 mov eax,dword ptr [ebp-18h] 00402354 mov dword ptr [ebp-10h],eax } 00402357 mov dword ptr [ebp-4],0FFFFFFFFh 0040235E lea ecx,[ebp-11h] 00402361 call CMyClass1::~CMyClass1 (4015D0h) 00402366 mov ecx,dword ptr [ebp-0Ch] 00402369 mov dword ptr fs:[0],ecx 00402370 pop ecx 00402371 mov esp,ebp 00402373 pop ebp 00402374 ret
可見函數2 加入了棧回退代碼。
因爲new 函數失敗時可能拋出異常，這時需要析構類變量s1，所以加入了棧回退


//沒有棧回退代碼 void cpp_exception_test3() { 004022D0 push ebp 004022D1 mov ebp,esp 004022D3 push ecx CMyClass1 s1; 004022D4 lea ecx,[s1] 004022D7 call CMyClass1::CMyClass1 (4015A0h) func_no_throw(); 004022DC call func_no_throw (4022C0h) } 004022E1 lea ecx,[s1] 004022E4 call CMyClass1::~CMyClass1 (4015D0h) 004022E9 mov esp,ebp 004022EB pop ebp 004022EC ret //有棧回退代碼 void cpp_exception_test4() { 004023B0 push ebp 004023B1 mov ebp,esp 004023B3 push 0FFFFFFFFh 004023B5 push offset __ehhandler$?ThreadFunc1@@YGKPAX@Z+9Ah (40267Ah) 004023BA mov eax,dword ptr fs:[00000000h] 004023C0 push eax 004023C1 push ecx 004023C2 mov eax,dword ptr [___security_cookie (40501Ch)] 004023C7 xor eax,ebp 004023C9 push eax 004023CA lea eax,[ebp-0Ch] 004023CD mov dword ptr fs:[00000000h],eax CMyClass1 s1; 004023D3 lea ecx,[ebp-0Dh] 004023D6 call CMyClass1::CMyClass1 (4015A0h) 004023DB mov dword ptr [ebp-4],0 func_has_throw(); 004023E2 call func_has_throw (4022F0h) } 004023E7 mov dword ptr [ebp-4],0FFFFFFFFh 004023EE lea ecx,[ebp-0Dh] 004023F1 call CMyClass1::~CMyClass1 (4015D0h) 004023F6 mov ecx,dword ptr [ebp-0Ch] 004023F9 mov dword ptr fs:[0],ecx 00402400 pop ecx 00402401 mov esp,ebp 00402403 pop ebp 00402404 ret
函數3 和 4 也同樣可以得到上面結論。


//沒有棧回退代碼 void cpp_exception_test5() { 00401CB0 push ebp 00401CB1 mov ebp,esp 00401CB3 push ecx throw 1; 00401CB4 mov dword ptr [ebp-4],1 00401CBB push offset __TI1H (402818h) 00401CC0 lea eax,[ebp-4] 00401CC3 push eax 00401CC4 call _CxxThrowException (401E74h) } 00401CC9 mov esp,ebp 00401CCB pop ebp 00401CCC ret
當函數拋出一個異常不會生成棧回退，因爲這裏沒有對象需要析構


//有棧回退代碼 void cpp_exception_test6() { 00401B10 push ebp 00401B11 mov ebp,esp 00401B13 push 0FFFFFFFFh 00401B15 push offset __ehhandler$?ThreadFunc1@@YGKPAX@Z+1Eh (401D3Eh) 00401B1A mov eax,dword ptr fs:[00000000h] 00401B20 push eax 00401B21 push ecx 00401B22 push ebx 00401B23 push esi 00401B24 push edi 00401B25 mov eax,dword ptr [___security_cookie (403004h)] 00401B2A xor eax,ebp 00401B2C push eax 00401B2D lea eax,[ebp-0Ch] 00401B30 mov dword ptr fs:[00000000h],eax 00401B36 mov dword ptr [ebp-10h],esp try { 00401B39 mov dword ptr [ebp-4],0 00401B40 jmp __catch$?cpp_exception_test6@@YAXXZ$0+6 (401B48h) } catch (...) 00401B42 mov eax,offset $LN2 (401B4Fh) 00401B47 ret { } 00401B48 mov dword ptr [ebp-4],0FFFFFFFFh } 00401B4F mov ecx,dword ptr [ebp-0Ch] 00401B52 mov dword ptr fs:[0],ecx 00401B59 pop ecx 00401B5A pop edi 00401B5B pop esi 00401B5C pop ebx 00401B5D mov esp,ebp 00401B5F pop ebp 00401B60 ret
當函數中有 try 語句時，不管有沒有對象需要析構，都會生成棧回退代碼  

總結：
編譯器會檢查函數中是否可能有異常發生，發生異常後是否需要完成棧回退(析構局部類變量)，或函數中是否有 try 語句
通過判斷決定是否加入異常處理代碼。