C++基础回顾-强制类型转换

最新推荐文章于 2025-08-15 18:53:39 发布

转载最新推荐文章于 2025-08-15 18:53:39 发布 · 103 阅读

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CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/SkyMouse/archive/2012/05/06/2485982.html

文章标签：

#c/c++

本文深入分析了C++中浮点数与整型数据的强制转换过程，通过汇编代码反向解析，揭示了1.0f与0.0f在转换时的差异，并解释了其背后的原因。

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直接上代码

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

请问输出结果如何?

1
1065353216
false
0
0
true

为什么0.0f和1.0f有这么大的差别呢?让我们反汇编看看代码如何?

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  //将1.0f装载到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 = a
0093164E  call        @ILT+340(__ftol2_sse) (931159h)  //具体做什么不太清楚好像望城了从float到long类型的转换,得到的值为1
00931653  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]  //输出a地址内容(强制将内容转换为int类型),a内存中的内容为3f800000(float的编码方式),强制转换为int类型得到了3f800000的10进制值
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; // 输出什么？
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)肯定输出false
009316BB  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; // 输出什么？
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对应内容为0x00000000,转换之后依然为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)

如此就可以推导出结果对应为

0x3f800000

false

true

转载于:https://www.cnblogs.com/SkyMouse/archive/2012/05/06/2485982.html