數(shù)據(jù)類型轉換

1、數(shù)據(jù)類型轉換一、隱式類型轉換1）簡單數(shù)據(jù)類型(1)算術運算轉換為最寬的數(shù)據(jù)類型eg:cpp view plain copy#include <iostream> using std:cout; using std:endl; int main(int argc, char* argv) int ival = 3; double dval = 3.14159; cout << ival + dval << endl;/ival被提升為double類型 return 0; 其運行結果：6.14159int main(int argc, char* argv)01

2、0D17D0 push ebp 010D17D1 mov ebp,esp 010D17D3 sub esp,0DCh 010D17D9 push ebx 010D17DA push esi 010D17DB push edi 010D17DC lea edi,ebp-0DCh 010D17E2 mov ecx,37h 010D17E7 mov eax,0CCCCCCCCh 010D17EC rep stos dword ptr es:edi int ival = 3;010D17EE mov dword ptr ival,3 double dval = 3.14159;010D17F5 mov

3、sd xmm0,mmword ptr _real400921f9f01b866e (010D6B30h) 010D17FD movsd mmword ptr dval,xmm0 cout << ival + dval << endl;/ival被提升為double類型010D1802 mov esi,esp 010D1804 push offset std:endl<char,std:char_traits<char> > (010D1064h) 010D1809 cvtsi2sd xmm0,dword ptr ival 010D180E add

4、sd xmm0,mmword ptr dval 010D1813 mov edi,esp 010D1815 sub esp,8 010D1818 movsd mmword ptr esp,xmm0 010D181D mov ecx,dword ptr _imp_?coutstd3V?$basic_ostreamDU?$char_traitsDstd1A (010D90A8h) 010D1823 call dword ptr _imp_std:basic_ostream<char,std:char_traits<char> >:operator<< (010D

5、90A0h) 010D1829 cmp edi,esp 010D182B call _RTC_CheckEsp (010D111Dh) 010D1830 mov ecx,eax 010D1832 call dword ptr _imp_std:basic_ostream<char,std:char_traits<char> >:operator<< (010D90A4h) 010D1838 cmp esi,esp 010D183A call _RTC_CheckEsp (010D111Dh) return 0;010D183F xor eax,eax 010

6、D1841 pop edi 010D1842 pop esi 010D1843 pop ebx 010D1844 add esp,0DCh 010D184A cmp ebp,esp 010D184C call _RTC_CheckEsp (010D111Dh) 010D1851 mov esp,ebp 010D1853 pop ebp 010D1854 ret (2)賦值轉換為被賦值對象的類型，但不會改變賦值對象的數(shù)據(jù)類型。eg:cpp view plain copy#include <iostream> using std:cout; using std:endl; int ma

7、in(int argc, char* argv) int ival = 4; double dval = 3.14159; ival = dval; / double->int cout << dval << endl; cout << ival << endl; return 0; 其運行結果：3.141593注意：數(shù)據(jù)類型窄化轉換時，注意數(shù)據(jù)溢出及丟失。(3)函數(shù)傳參 當實參與形參數(shù)據(jù)類型不同時，轉換為形參數(shù)據(jù)類型。eg:cpp view plain copy#include <iostream> using std:cou

8、t; using std:endl; double square(double dval); int main(int argc, char* argv) cout << square(5) << endl; return 0; double square(double dval) return dval * dval; 其運行結果：25(4)函數(shù)返回當返回類型與表達式類型不同時，轉換為返回類型。eg:cpp view plain copy#include <iostream> using std:cout; using std:endl; double d

9、ifference(int ival1, int ival2); int main(int argc, char* argv) int ival1 = 2; int ival2 = 3; cout << difference(2, 3) << endl; return 0; double difference(int ival1, int ival2) return ival1 - ival2; /返回值被提升為double類型 其運行結果：-12)類類型(1)單參數(shù)構造函數(shù)(2)賦值操作符(3)類型轉換操作符eg:cpp view plain copy/ implic

10、it conversion of classes: #include <iostream> using std:cout; using std:endl; class A ; class B public: / conversion from A (constructor) B(const A& x) cout << "Conversion from A (constructor)" << endl; / conversion from A (assignment) B& operator= (const A& x

11、) cout << "Conversion from A (assignment)" << endl; return *this; / conversion to A (type-cast operator) operator A() cout << "Conversion to A (type-cast operator)" << endl; return A(); ; int main(int argc, char* argv) A foo; B bar = foo; / calls construct

12、or bar = foo; / calls assignment foo = bar; / calls type-cast operator return 0; 其運行結果：Conversion from A (constructor)Conversion from A (assignment)Conversion to A (type-cast operator)二、顯示類型轉換1)C風格dst = (T)srceg：cpp view plain copy#include <iostream> using std:cout; using std:endl; int main(in

13、t argc, char* argv) int ival; double dval = 3.14159; ival = (int)dval; / double->int cout << dval << endl; cout << ival << endl; return 0; 其運行結果：3.1415932)函數(shù)風格dst = T(src)eg：cpp view plain copy#include <iostream> using std:cout; using std:endl; int main(int argc, cha

14、r* argv) int ival; double dval = 3.14159; ival = int(dval); / double->int cout << dval << endl; cout << ival << endl; return 0; 其運行結果：3.1415933)(1)static_casta、 類層次結構中基類和派生類之間指針或者引用的轉換。up-casting (把派生類的指針或引用轉換成基類的指針或者引用表示)是安全的；down-casting(把基類指針或引用轉換成子類的指針或者引用)是不安全的。b、基本數(shù)據(jù)

15、類型之間的轉換c、把空指針轉換成目標類型的空指針(null pointer)d、 把任何類型的表達式轉換成void類型注意：不能轉換掉表達式的const、volitale或者_unaligned屬性。eg1:cpp view plain copy#include <iostream> using std:cout; using std:endl; class Dummy double i, j; ; class Addition int x, y; public: Addition(int a, int b) x = a; y = b; int result() return x

16、+ y; ; int main(int argc, char* argv) Dummy d; Addition * padd; padd = (Addition*)&d; cout << padd->result(); return 0; 其運行結果：-1717986920不做類型檢查，轉換沒有安全性eg2：cpp view plain copy#include <iostream> using std:cout; using std:endl; class Dummy double i, j; ; class Addition int x, y; pub

17、lic: Addition(int a, int b) x = a; y = b; int result() return x + y; ; int main(int argc, char* argv) Dummy d; Addition * padd; padd = static_cast<Addition*>(&d); cout << padd->result(); return 0; (2)dynamic_cast轉換類型與表達式類型相同，必須同時是類的指針、類的引用或void *.用于類的上行、下行及交叉轉換。一般情況下，dynamic_cast用

18、于具有多態(tài)性的類(即有虛函數(shù)的類)的類型轉換。a、上行轉換時，其與static_cast效果相同；b、下行轉換時，其具有類型轉換的功能，比static_cast更安全；c、交叉轉換時，其轉換成空指針，而static_cast則不允許轉換。注意：不能轉換掉表達式的const、volitale或者_unaligned屬性。eg:cpp view plain copy/ dynamic_cast #include <iostream> #include <exception> using std:cout; using std:endl; using std:exceptio

19、n; class Base virtual void dummy() ; class Derived : public Base int a; ; int main(int argc, char* argv) try Base * pba = new Derived; Base * pbb = new Base; Derived * pd; pd = dynamic_cast<Derived*>(pba); if (pd = 0) cout << "Null pointer on first type-cast.n" pd = dynamic_cas

20、t<Derived*>(pbb); if (pd = 0) cout << "Null pointer on second type-cast.n" catch (exception& e) cout << "Exception: " << e.what(); return 0; 其運行結果：Null pointer on second type-cast.(3)reinterpret_cast轉換一個指針為其他類型的指針，也允許將一個指針轉換為整數(shù)類型，反之亦然。這個操作符能夠在非相關的類型之間進

21、行轉換。操作結果只是簡單的從一個指針到別的指針的值的二進制拷貝，在類型之間指向的內(nèi)容不做任何類型的檢查和轉換。這是一個強制轉換。使用時有很大的風險，慎用之。注意：不能轉換掉表達式的const、volitale或者_unaligned屬性。eg1:cpp view plain copy/ expre_nterpret_cast_Operator.cpp / compile with: /EHsc #include <iostream> using std:cout; using std:endl; / Returns a hash code based on an address u

22、nsigned short Hash(void *p) unsigned int val = reinterpret_cast<unsigned int>(p); return (unsigned short)(val (val >> 16); int main(int argc, char* argv) int a20; for (int i = 0; i < 20; i+) cout << Hash(a + i) << endl; eg2:cpp view plain copy#include <iostream> stru

23、ct Foo ; struct Bar ; int main() Foo* f = new Foo; Bar* b1 = f; Bar* b2 = static_cast<Bar*>(f); Bar* b3 = dynamic_cast<Bar*>(f); Bar* b4 = const_cast<Bar*>(f); Bar* b5 = reinterpret_cast<Bar*>(f); return 0; 1>- 已啟動全部重新生成: 項目: test, 配置: Debug Win32 -1> main.cpp1>f:wor

24、kspacetesttestmain.cpp(10): error C2440: “初始化”: 無法從“Foo *”轉換為“Bar *”1> f:workspacetesttestmain.cpp(10): note: 與指向的類型無關；轉換要求 reinterpret_cast、C 樣式轉換或函數(shù)樣式轉換1>f:workspacetesttestmain.cpp(11): error C2440: “static_cast”: 無法從“Foo *”轉換為“Bar *”1> f:workspacetesttestmain.cpp(11): note: 與指向的類型無關；轉換要

25、求 reinterpret_cast、C 樣式轉換或函數(shù)樣式轉換1>f:workspacetesttestmain.cpp(12): error C2683: “dynamic_cast”:“Foo”不是多態(tài)類型1> f:workspacetesttestmain.cpp(3): note: 參見“Foo”的聲明1>f:workspacetesttestmain.cpp(13): error C2440: “const_cast”: 無法從“Foo *”轉換為“Bar *”1> f:workspacetesttestmain.cpp(13): note: 與指向的類型無

26、關；轉換要求 reinterpret_cast、C 樣式轉換或函數(shù)樣式轉換= 全部重新生成: 成功 0 個，失敗 1 個，跳過 0 個 =(4)const_cast修改類型的const或volatile屬性a、常量指針被轉化成非常量指針，轉換后指針指向原來的變量(即轉換后的指針地址不變)eg1:cpp view plain copy/ const_cast #include <iostream> using std:cout; using std:endl; void print(char * str) cout << str << endl; int ma

27、in(int argc, char* argv) const char * c = "sample text" print(const_cast<char *> (c); return 0; 其運行結果：sample texteg2:cpp view plain copy#include <iostream> using std:cout; using std:endl; class A public: A() m_iNum = 0; int m_iNum; ; int main(int argc, char* argv) /1、指針指向類 cons

28、t A *pca1 = new A; A *pa2 = const_cast<A*>(pca1); /常量對象轉換為非常量對象 pa2->m_iNum = 200; /轉換后指針指向原來的對象 cout << pca1->m_iNum << " " << pa2->m_iNum << endl; /200 200 /2、指針指向基本類型 const int ica = 100; int * ia = const_cast<int *>(&ica); *ia = 200; cou

29、t << *ia << " " << ica << endl; /200 100 return 0; 其運行結果：200 200200 100b、常量引用轉為非常量引用eg:cpp view plain copy#include <iostream> using std:cout; using std:endl; class A public: A() m_iNum = 1; int m_iNum; ; int main(int argc, char* argv) A a0; const A &a1 = a0; A a2 = const_cast<A&>(a1); /常量引用轉為非常量引用 a2.m_iNum = 200; cout << a0.m_iNum << endl; cout << a1.m_iNum << endl; cout << a2.m_iNum << endl; return 0; 其運行結果：11200c、