C学习笔记(自增)
自增
(1)后缀:与Turbo C相同,在语句结束之前或者说分号之前才会执行自增。
(2)前缀:
前两个自增统一取值,后面的自增即为取值。
int i=2,j; j=++i+(++i)+(++i);
j=4+4+5;
注意不能直接写j=++i+++i+++i;
否则会报错error C2105: '++' needs l-value;
因为++后面跟的还是+,必须要左值才能++;
int i=2,j; j=(i++)++;
如此也同样会报错error C2105: '++' needs l-value;
因为i++返回的是一个数值 ,而不是一个可赋值的变量(左值);
统一取值必须是指前两个并且必须是连在一起的,否则每一个都是自增即为取值。
int i=2,j; j=++i+i+++(++i)+(++i);
j=3+3+4+5;
无论是int 还是 float都遵循这个规则。
(3)在乘法中的使用规则:
i++*++i自动识别为++i*i++;
int i=2,j1,j2; j1=i++*++i; i=2; j2=++i*i++;
j1=3*3;
j1==j2;
自减也是同样的。
在printf中,后缀自增也是在语句结束是才进行操作的。
int i=2; printf("%d\n",i=i++); i=2; printf("%d %d\n",i,i++);
结果都是2。
《裘宗燕:C/C++ 语言中的表达式求值》文章摘要:
另外,想要知道变量更新的值是否已经保存到了内存,需要注意顺序点在哪里;
C/C++语言定义(语言的参考手册)明确定义了顺序点的概念。顺序点位于:
1. 每个完整表达式结束时。完整表达式包括变量初始化表达式,表达式语句,return语句的表达式,以及条件、循环和switch语句的控制表达式(for头部有三个控制表达式);
2. 运算符 &&、||、?: 和逗号运算符的第一个运算对象计算之后;
3. 函数调用中对所有实际参数和函数名表达式(需要调用的函数也可能通过表达式描述)的求值完成之后(进入函数体之前)。
而且C/C++并没有对所有的运算顺序进行规定,而是交由编译器根据需要实现调整,从而得到效率更高的代码;
Java则有严格规定表达式求值顺序,但大部分程序设计语言实际上都才用了类似C/C++的规定。
“谁知道下面C语句给n赋什么值?”
m = 1; n = m++ +m++;
正确回答是:不知道!语言没有规定它应该算出什么,结果完全依赖具体系统在具体上下文中的具体处理。
其中牵涉到运算对象的求值顺序和变量修改的实现时刻问题。
评论区摘要:
②c99:
Annex J
J.1 Unspecified behavior:
— The order in which subexpressions are evaluated and the order in which side effects take place,
except as specified for the function-call (), &&, ||, ?:, and comma operators (6.5).
— The order in which the function designator(操作数指示符), arguments(实参), and subexpressions(子表达式) within the arguments are evaluated in a function call (6.5.2.2).
— The order in which the operands of an assignment operator(赋值运算符的操作数) are evaluated(被求值) (6.5.16).
........
J.2 Undefined behavior
— Between two sequence points(序列点/顺序点), an object is modified(修正) more than once, or is modified
and the prior value is read other than(除了) to determine the value to be stored (6.5).
......
④c++0x最终草案(FDIS )
ISO/IEC FDIS 14882
N3290:
1.9 Program execution(执行)
15 Except where noted, evaluations of operands of individual operators and of subexpressions of individual
expressions are unsequenced. [ Note: In an expression that is evaluated more than once during the execution
of a program,unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed
consistently(一贯地)in different evaluations. —end note ] The value computations of the operands of
an operator are sequenced before the value computation of the result of the operator. If a side effect on a
scalar object(标量对象)is unsequenced relative to either another side effect on the same scalar object or
a value computation using the value of the same scalar object, the behavior is undefined.
[ Example:
void f(int, int);
void g(int i, int* v) {
i = v[i++]; // the behavior is undefined
i = 7, i++, i++; // i becomes 9
i = i++ + 1; // the behavior is undefined
i = i + 1; // the value of i is incremented(递增)
f(i = -1, i = -1); // the behavior is undefined
}
—end example ]
When calling a function (whether or not the function is inline(内联)[内联函数继承了宏定义的优点,也消除了它的缺点]),
every value computation and side effect associated with any argument expression, or with the postfix expression
designating(把...定名为) the called function, is sequenced before execution of every
expression or statement in the body of the called function. [ Note: Value computations and side effects
associated with different argument expressions are
unsequenced. —end note.]
Every evaluation in the calling function (including other function calls) that is not otherwise specifically
sequenced before or after the execution of the body
of the called function is indeterminately sequenced with respect to the execution of the called function.9 Several
contexts in C++ cause evaluation of a function call,even though no corresponding function call syntax appears i
n the translation unit.
[ Example: Evaluation of a new expression invokes(借助) one or more allocation and constructor functions; see 5.3.4.
For another example,invocation of a conversion function (12.3.2) can arise in contexts(上下文)in which no function call
syntax appears.—end example ] The sequencing constraints(约束) on the execution of the called function (as described above)
are features of the function calls as evaluated, whatever the syntax of the expression that calls the function might be.
总结:尽量不要写容易发生误会的表达式,因为运算顺序本身就没有严格规定,所以不要迷信编译器,不要迷信执行结果,有些事情确实是无法定论的。所以之前总结的运算规律,
也只适合考试的时候看看,要真正掌握原理,还是需要用VC时按Alt+8、Alt+6和Alt+5,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(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)。
而且有时候老师说的东西也是不准确的,面对什么问题,都需要有一种刨根问底的精神。
参考:http://blog.csdn.net/chenbang110/article/details/9192595
参考:http://bbs.csdn.net/topics/370153775