无符号整数和带符号整数运算问题？
我看到一本书上将的数据类型转换的规则：
（5）、否则，如果带符号整数类型的操作数级别低于无符号整数类型的级别，就把带符号整数类型的操作数转换为无符号整数类型。
（6）、否则，如果带符号整数类型的值域包含了无符号整数类型所表示的值，就把无符号整数类型转换为带符号整数类型。
可是，今天在遇到这样一个问题
unsigned int a=10;
if(a-12>0)
prinf("aaaa");
这个永远都是成立的，也就是说 a-12的结果是一个无符号整数，参照上面的第6条，就出现不一样了，到底是怎么样的，求指导！
[解决办法]
unsigned int a=10; int i = 12; short s = 12; long long ll = 12; if(a-i>0) cout<<("u-i")<<endl; if(a-s>0) cout<<"u-s"<<endl; if(a-ll>0) cout<<"u-ll"<<endl;
[解决办法]
A.6.5 Arithmetic Conversions
Many operators cause conversions and yield result types in a similar way. The effect is to bring operands into a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions.
First, if either operand is long double, the other is converted to long double.
Otherwise, if either operand is double, the other is converted to double.
Otherwise, if either operand is float, the other is converted to float.
Otherwise, the integral promotions are performed on both operands; then, if either operand is unsigned long int, the other is converted to unsigned long int.
Otherwise, if one operand is long int and the other is unsigned int, the effect depends on whether a long int can represent all values of an unsigned int; if so, the unsigned int operand is converted to long int; if not, both are converted to unsigned long int.
Otherwise, if one operand is long int, the other is converted to long int.
Otherwise, if either operand is unsigned int, the other is converted to unsigned int.
Otherwise, both operands have type int.
There are two changes here. First, arithmetic on float operands may be done in single precision, rather than double; the first edition specified that all floating arithmetic was double precision. Second, shorter unsigned types, when combined with a larger signed type, do not propagate the unsigned property to the result type; in the first edition, the unsigned always dominated. The new rules are slightly more complicated, but reduce somewhat the surprises that may occur when an unsigned quantity meets signed. Unexpected results may still occur when an unsigned expression is compared to a signed expression of the same size.
[解决办法]
烂书误人
[解决办法]
这个没错。两边都是无符号整数，结果当然也是无符号整数。
[解决办法]
VC调试(TC或BC用TD调试)时按Alt+8、Alt+6和Alt+5,打开汇编窗口、内存窗口和寄存器窗口看每句C对应的汇编、单步执行并观察相应内存和寄存器变化，这样过一遍不就啥都明白了吗。
对VC来说，所谓‘调试时’就是编译连接通过以后，按F10或F11键单步执行一步以后的时候，或者在某行按F9设了断点后按F5执行停在该断点处的时候。
（Linux或Unix下可以在用GDB调试时,看每句C对应的汇编并单步执行观察相应内存和寄存器变化。）
提醒：
“学习用汇编语言写程序”
和
“VC调试(TC或BC用TD调试)时按Alt+8、Alt+6和Alt+5,打开汇编窗口、内存窗口和寄存器窗口看每句C对应的汇编、单步执行并观察相应内存和寄存器变化，这样过一遍不就啥都明白了吗。
（Linux或Unix下可以在用GDB调试时,看每句C对应的汇编并单步执行观察相应内存和寄存器变化。）”
不是一回事！
不要迷信书、考题、老师、回帖；
要迷信CPU、编译器、调试器、运行结果。
并请结合“盲人摸太阳”和“驾船出海时一定只带一个指南针。”加以理解。
任何理论、权威、传说、真理、标准、解释、想象、知识……都比不上摆在眼前的事实！
不要写连自己也预测不了结果的代码！
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