C++基礎回顧-強制類型轉換

直接上代碼

float a = 1.0f;cout << (int)a << endl;cout << (int&)a << endl;cout << boolalpha << ( (int)a == (int&)a ) << endl; // 輸出什嗎？float b = 0.0f;cout << (int)b << endl;cout << (int&)b << endl;cout << boolalpha << ( (int)b == (int&)b ) << endl; // 輸出什嗎？

請問輸出結果如何?

11065353216false00true

為什麼0.0f和1.0f有這麼大的差別呢?讓我們反組譯碼看看代碼如何?

00931620  push        ebp  00931621  mov         ebp,esp  00931623  sub         esp,0D8h  00931629  push        ebx  0093162A  push        esi  0093162B  push        edi  0093162C  lea         edi,[ebp-0D8h]  00931632  mov         ecx,36h  00931637  mov         eax,0CCCCCCCCh  0093163C  rep stos    dword ptr es:[edi]  float a = 1.0f;0093163E  fld1  //將1.0f裝載到st(0)00931640  fstp        dword ptr [a]  cout << (int)a << endl;00931643  mov         esi,esp  00931645  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  0093164A  push        eax  0093164B  fld         dword ptr [a]  //st0 = a0093164E  call        @ILT+340(__ftol2_sse) (931159h)  //具體做什麼不太清楚好像望城了從float到long類型的轉換,得到的值為100931653  mov         edi,esp  00931655  push        eax  00931656  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  0093165C  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D314h)]  00931662  cmp         edi,esp  00931664  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  00931669  mov         ecx,eax  0093166B  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  00931671  cmp         esi,esp  00931673  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  cout << (int&)a << endl;00931678  mov         esi,esp  0093167A  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  0093167F  push        eax  00931680  mov         edi,esp  00931682  mov         ecx,dword ptr [a]  //輸出a地址內容(強制將內容轉換為int類型),a記憶體中的內容為3f800000(float的編碼方式),強制轉換為int類型得到了3f800000的10進位值00931685  push        ecx  00931686  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  0093168C  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D314h)]  00931692  cmp         edi,esp  00931694  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  00931699  mov         ecx,eax  0093169B  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  009316A1  cmp         esi,esp  009316A3  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  cout << boolalpha << ( (int)a == (int&)a ) << endl; // 輸出什嗎？009316A8  mov         esi,esp  009316AA  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  009316AF  push        eax  009316B0  fld         dword ptr [a] 009316B3  call        @ILT+340(__ftol2_sse) (931159h)  009316B8  cmp         eax,dword ptr [a]  //(1==0x3f800000)肯定輸出false009316BB  sete        cl  009316BE  mov         edi,esp  009316C0  movzx       edx,cl  009316C3  push        edx  009316C4  mov         ebx,esp  009316C6  push        offset std::boolalpha (931113h)  009316CB  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  009316D1  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D30Ch)]  009316D7  cmp         ebx,esp  009316D9  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  009316DE  mov         ecx,eax  009316E0  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D308h)]  009316E6  cmp         edi,esp  009316E8  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  009316ED  mov         ecx,eax  009316EF  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  009316F5  cmp         esi,esp  009316F7  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  float b = 0.0f;009316FC  fldz  009316FE  fstp        dword ptr [b]  cout << (int)b << endl;00931701  mov         esi,esp  00931703  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  00931708  push        eax  00931709  fld         dword ptr [b]  0093170C  call        @ILT+340(__ftol2_sse) (931159h)  00931711  mov         edi,esp  00931713  push        eax  00931714  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  0093171A  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D314h)]  00931720  cmp         edi,esp  00931722  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  00931727  mov         ecx,eax  00931729  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  0093172F  cmp         esi,esp  00931731  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  cout << (int&)b << endl;00931736  mov         esi,esp  00931738  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  0093173D  push        eax  0093173E  mov         edi,esp  00931740  mov         ecx,dword ptr [b]  00931743  push        ecx  00931744  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  0093174A  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D314h)]  00931750  cmp         edi,esp  00931752  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  00931757  mov         ecx,eax  00931759  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  0093175F  cmp         esi,esp  00931761  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  cout << boolalpha << ( (int)b == (int&)b ) << endl; // 輸出什嗎？00931766  mov         esi,esp  00931768  mov         eax,dword ptr [__imp_std::endl (93D31Ch)]  0093176D  push        eax  0093176E  fld         dword ptr [b]  00931771  call        @ILT+340(__ftol2_sse) (931159h)  00931776  cmp         eax,dword ptr [b]  //b對應內容為0x00000000,轉換之後依然為0所以相等00931779  sete        cl  0093177C  mov         edi,esp  0093177E  movzx       edx,cl  00931781  push        edx  00931782  mov         ebx,esp  00931784  push        offset std::boolalpha (931113h)  00931789  mov         ecx,dword ptr [__imp_std::cout (93D318h)]  0093178F  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D30Ch)]  00931795  cmp         ebx,esp  00931797  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  0093179C  mov         ecx,eax  0093179E  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D308h)]  009317A4  cmp         edi,esp  009317A6  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  009317AB  mov         ecx,eax  009317AD  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (93D310h)]  009317B3  cmp         esi,esp  009317B5  call        @ILT+505(__RTC_CheckEsp) (9311FEh)  

如此就可以推匯出結果對應為

1

0x3f800000

false

0

0　　

true