Operators

• ->
  • it combine the dereference and member access.
  • 引用成员 -
• .
  • 访问结构成员
  • return 这个成员
• []
  • ^eaba19
• ()
  • call operator，用于启用函数
• - ^4ded86
  • The minus sign is an operator that negates the value of its (literal) operand.
• 位运算
  • 位段

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• :: ^a798ff
  • scope operator「resolver 域的解析符」, used to access names in a namespace
  • when it has an empty left-hand side, it is a request to fetch the name on the right-hand side from the global scope.

Arithmetic

• left associative
• yield rvalue
• the operands to % must have integral type

Logical and Relational

• require and yield rvalue.

Assignment

• right associative and yield left-hand operand as lvalue. (可以使用连等号，但注意规矩，连起来的要么相同，要么可转化)
• if the right-hand operand is a braced initializer list (a curly brace) and it contain at most one value, that value doesn’t allowed a narrowing conversion.

Brevity Can Be a Virtue code-style

int i = get_value();    // get first value
while(i != 42){
  /* statements */
  i = get_value();  // get next value
}

int i;
while((i = get_value()) != 42){
  /* statements */
}

Increment and Decrement

• 符号在前，先加的值；符号在后，先值再加。

int i = 0, j;
j = ++i;  // j=1, i=1
j = i++;  // j=1, i=2

• prefix version yields lvalue, postfix version yields rvalue.
• Better to Use Prefix Version. code-style 因为后缀版会有一个保存原值的行为，而当使用复杂的iterator时，这个开销是很大的

Brevity Can Be a Virtue code-style

cout << *pbeg << endl;
pbeg++;

cout << *pbeg++ << endl;

Conditional

cond ? expr1 : expr2

• guarantees only one of the expr is evaluated.
• result lvalue if the expr is lvalue.
• 优先级可低了，通常都是跟括号一起出现的

Bitwise

• take operands of integral type
• As usual, “small integer” will first promoted
• operands can be either signed or unsigned, but the way to handle “sign bit” is machine dependent. So, better use unsigned type

一些小技巧 (C++ Primer, p155)

获取某位on，不要算，而是

1UL << 27

on某一位，用or；off某一位，用and

quiz1 |= 1UL << 27;
quiz1 &= ~(1UL << 27);

and和or都有保证别的不动，其中一个必动的方法，而且刚好这个必动是相反的

判断某一位是否on

bool status = quiz1 & (1UL << 27);

结合前面的判断，这里还可以有个or的反逻辑

sizeof

• result is a constant expression (编译时就决定了的值), type of size_t. (也因此可以用做定义数组的维数)
• doesn’t evaluate its operand
• doesn’t convert the array to a pointer

以前的笔记

首先, 这不是函数, 是一个运算符

给出某个 类型 或 变量 在内存中所占的 字节数

int a=6;
 
printf("sizeof(int)=%d\n",sizeof(int));			// sizeof(int)=4
printf("sizeof(a)=%d\n",sizeof(a));				// sizeof(a)=4
printf("sizeof(double)=%d\n",sizeof(double));	// sizeof(double)=8

sizeof是静态运算符, 它的结果在编译时刻就决定了 就是别在里头做运算, 不会算的

int a=6;
 
printf("sizeof(a++)=%d\n",sizeof(a++));			// sizeof(a++)=4
printf("sizeof(a+1.0)=%d\n",sizeof(a+1.0));		// sizeof(a+1.0)=8

在编译的时候, 编译器读到sizeof就会看括号里的东西是多少字节, 再把这个数将整个sizeof替换掉, 所以在运行的时候, 括号里的表达式是没被运行的

Comma

• guarantee the Order of Evaluate: left to right
• 左侧表达式的结果被舍掉了，result是右侧表达式。whether lvalue depand on the r-h operand.