数据类型转换

1、数据类型转换一、隐式类型转换1）简单数据类型(1)算术运算转换为最宽的数据类型eg:cpp view plain copy#include using std:cout; using std:endl; int main(int argc, char* argv) int ival = 3; double dval = 3.14159; cout ival + dval endl;/ival被提升为double类型 return 0; 其运行结果：6.14159int main(int argc, char* argv)010D17D0 push ebp 010D17D1 mov ebp,es

2、p 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 movsd xmm0,mmword ptr _real400921f9f01

3、b866e (010D6B30h) 010D17FD movsd mmword ptr dval,xmm0 cout ival + dval endl;/ival被提升为double类型010D1802 mov esi,esp 010D1804 push offset std:endlchar,std:char_traits (010D1064h) 010D1809 cvtsi2sd xmm0,dword ptr ival 010D180E addsd xmm0,mmword ptr dval 010D1813 mov edi,esp 010D1815 sub esp,8 010D1818 m

4、ovsd mmword ptr esp,xmm0 010D181D mov ecx,dword ptr _imp_?coutstd3V?$basic_ostreamDU?$char_traitsDstd1A (010D90A8h) 010D1823 call dword ptr _imp_std:basic_ostreamchar,std:char_traits :operator (010D90A0h) 010D1829 cmp edi,esp 010D182B call _RTC_CheckEsp (010D111Dh) 010D1830 mov ecx,eax 010D1832 call

5、 dword ptr _imp_std:basic_ostreamchar,std:char_traits :operator (010D90A4h) 010D1838 cmp esi,esp 010D183A call _RTC_CheckEsp (010D111Dh) return 0;010D183F xor eax,eax 010D1841 pop edi 010D1842 pop esi 010D1843 pop ebx 010D1844 add esp,0DCh 010D184A cmp ebp,esp 010D184C call _RTC_CheckEsp (010D111Dh)

6、 010D1851 mov esp,ebp 010D1853 pop ebp 010D1854 ret (2)赋值转换为被赋值对象的类型，但不会改变赋值对象的数据类型。eg:cpp view plain copy#include using std:cout; using std:endl; int main(int argc, char* argv) int ival = 4; double dval = 3.14159; ival = dval; / double-int cout dval endl; cout ival endl; return 0; 其运行结果：3.141593注意：

7、数据类型窄化转换时，注意数据溢出及丢失。(3)函数传参 当实参与形参数据类型不同时，转换为形参数据类型。eg:cpp view plain copy#include using std:cout; 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; 其运行结果：25(4)函数返回当返回类型与表达式类型不同时，转换为返回类型。eg:cpp vie

8、w plain copy#include using std:cout; using std:endl; double difference(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; /返回值被提升为double类型 其运行结果：-12)类类型(1)单参数构造函数(2)赋值

9、操作符(3)类型转换操作符eg:cpp view plain copy/ implicit conversion of classes: #include 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) cout Conversio

10、n 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 constructor bar = foo; / calls assignment foo = bar; / calls type-cast operator retur

11、n 0; 其运行结果：Conversion from A (constructor)Conversion from A (assignment)Conversion to A (type-cast operator)二、显示类型转换1)C风格dst = (T)srceg：cpp view plain copy#include using std:cout; using std:endl; int main(int argc, char* argv) int ival; double dval = 3.14159; ival = (int)dval; / double-int cout dval

12、 endl; cout ival endl; return 0; 其运行结果：3.1415932)函数风格dst = T(src)eg：cpp view plain copy#include using std:cout; using std:endl; int main(int argc, char* argv) int ival; double dval = 3.14159; ival = int(dval); / double-int cout dval endl; cout ival endl; return 0; 其运行结果：3.1415933)(1)static_casta、 类层

13、次结构中基类和派生类之间指针或者引用的转换。up-casting (把派生类的指针或引用转换成基类的指针或者引用表示)是安全的；down-casting(把基类指针或引用转换成子类的指针或者引用)是不安全的。b、基本数据类型之间的转换c、把空指针转换成目标类型的空指针(null pointer)d、 把任何类型的表达式转换成void类型注意：不能转换掉表达式的const、volitale或者_unaligned属性。eg1:cpp view plain copy#include using std:cout; using std:endl; class Dummy double i, j; ;

14、 class Addition int x, y; public: 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 = (Addition*)&d; cout result(); return 0; 其运行结果：-1717986920不做类型检查，转换没有安全性eg2：cpp view plain copy#include using std:cout; using std:endl; c

15、lass Dummy double i, j; ; class Addition int x, y; public: 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(&d); cout result(); return 0; (2)dynamic_cast转换类型与表达式类型相同，必须同时是类的指针、类的引用或void *.用于类的上行、下行及交叉转换。一般情况

16、下，dynamic_cast用于具有多态性的类(即有虚函数的类)的类型转换。a、上行转换时，其与static_cast效果相同；b、下行转换时，其具有类型转换的功能，比static_cast更安全；c、交叉转换时，其转换成空指针，而static_cast则不允许转换。注意：不能转换掉表达式的const、volitale或者_unaligned属性。eg:cpp view plain copy/ dynamic_cast #include #include using std:cout; using std:endl; using std:exception; class Base virtua

17、l 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(pba); if (pd = 0) cout Null pointer on first type-cast.n; pd = dynamic_cast(pbb); if (pd = 0) cout Null pointer on second type-cas

18、t.n; catch (exception& e) cout Exception: e.what(); return 0; 其运行结果：Null pointer on second type-cast.(3)reinterpret_cast转换一个指针为其他类型的指针，也允许将一个指针转换为整数类型，反之亦然。这个操作符能够在非相关的类型之间进行转换。操作结果只是简单的从一个指针到别的指针的值的二进制拷贝，在类型之间指向的内容不做任何类型的检查和转换。这是一个强制转换。使用时有很大的风险，慎用之。注意：不能转换掉表达式的const、volitale或者_unaligned属性。eg1:cpp

19、view plain copy/ expre_nterpret_cast_Operator.cpp / compile with: /EHsc #include using std:cout; using std:endl; / Returns a hash code based on an address unsigned short Hash(void *p) unsigned int val = reinterpret_cast(p); return (unsigned short)(val (val 16); int main(int argc, char* argv) int a20

20、; for (int i = 0; i 20; i+) cout Hash(a + i) endl; eg2:cpp view plain copy#include struct Foo ; struct Bar ; int main() Foo* f = new Foo; Bar* b1 = f; Bar* b2 = static_cast(f); Bar* b3 = dynamic_cast(f); Bar* b4 = const_cast(f); Bar* b5 = reinterpret_cast(f); return 0; 1- 已启动全部重新生成: 项目: test, 配置: De

21、bug Win32 -1 main.cpp1f:workspacetesttestmain.cpp(10): error C2440: “初始化”: 无法从“Foo *”转换为“Bar *”1 f:workspacetesttestmain.cpp(10): note: 与指向的类型无关；转换要求 reinterpret_cast、C 样式转换或函数样式转换1f:workspacetesttestmain.cpp(11): error C2440: “static_cast”: 无法从“Foo *”转换为“Bar *”1 f:workspacetesttestmain.cpp(11): not

22、e: 与指向的类型无关；转换要求 reinterpret_cast、C 样式转换或函数样式转换1f:workspacetesttestmain.cpp(12): error C2683: “dynamic_cast”:“Foo”不是多态类型1 f:workspacetesttestmain.cpp(3): note: 参见“Foo”的声明1f:workspacetesttestmain.cpp(13): error C2440: “const_cast”: 无法从“Foo *”转换为“Bar *”1 f:workspacetesttestmain.cpp(13): note: 与指向的类型无关

23、；转换要求 reinterpret_cast、C 样式转换或函数样式转换= 全部重新生成: 成功 0 个，失败 1 个，跳过 0 个 =(4)const_cast修改类型的const或volatile属性a、常量指针被转化成非常量指针，转换后指针指向原来的变量(即转换后的指针地址不变)eg1:cpp view plain copy/ const_cast #include using std:cout; using std:endl; void print(char * str) cout str endl; int main(int argc, char* argv) const char

24、* c = sample text; print(const_cast (c); return 0; 其运行结果：sample texteg2:cpp view plain copy#include using std:cout; using std:endl; class A public: A() m_iNum = 0; int m_iNum; ; int main(int argc, char* argv) /1、指针指向类 const A *pca1 = new A; A *pa2 = const_cast(pca1); /常量对象转换为非常量对象 pa2-m_iNum = 200; /转换后指针指向原来的对象 cout m_iNum m_iNum endl; /200 200 /2、指针指向基本类型 const int ica = 100; int * ia = const_cast(&ica); *ia = 200; cout *ia ica endl; /200 100 return 0; 其运行结果：200 200200 100b、常量引用转为非常量引用eg:cpp view plain copy#include using std:cout; using std:endl; class A public: A() m_iNum