level 5
muyexiudong
楼主
文章上说c++中把负值赋给unsigned对象是完全合法的，其结果是该负数对该类型的取值个数求模后的值。所以，如果把-1赋给8位的unsigned char那么结果是255.因为255是-1对256求模后的值求解
2012年03月15日 04点03分
1
可是我用-1%256运行为-1
2012年03月15日 04点03分
level 5
muyexiudong
楼主
3
level 5
muyexiudong
楼主
#include<iostream>
using namespace std;
int main ()
{
cout<<-1%256<<endl;
return 0;
}
2012年03月15日 04点03分
5
using namespace std;
int main ()
{
cout<<-1%256<<endl;
return 0;
}
level 5
level 5
level 5
muyexiudong
楼主
#include<iostream>
using namespace std;
int main ()
{ unsigned char i=-1;
cout<<i<<endl;
return 0;
}我用编译器运行了下
无结果显示
2012年03月15日 04点03分
11
using namespace std;
int main ()
{ unsigned char i=-1;
cout<<i<<endl;
return 0;
}我用编译器运行了下
无结果显示
level 5
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level 15
level 15
类型表示不是implementation-defined behavior，但实际上选择哪一种是至少unspecified。
ISO C++11
3.9.1
7 Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.48 A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system.49 [ Example: this International Standard permits 2’s complement, 1’s complement and signed magnitude representations for integral types. —end example ]
而无符号整数向有符号数转换是明确的implementation-defined behavior：
ISO C++11
4.7
3 If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bit-field width); otherwise, the value is implementation-defined.
Index of implementation-defined behavior
value of result of unsigned to signed conversion, 84
2012年03月15日 05点03分
20
ISO C++11
3.9.1
7 Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.48 A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system.49 [ Example: this International Standard permits 2’s complement, 1’s complement and signed magnitude representations for integral types. —end example ]
而无符号整数向有符号数转换是明确的implementation-defined behavior：
ISO C++11
4.7
3 If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bit-field width); otherwise, the value is implementation-defined.
Index of implementation-defined behavior
value of result of unsigned to signed conversion, 84