变量是左值，因此可以出现在赋值语句的左边。数字字面值是右值，因此不能被赋值。给定以下变量。
Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned. Given the variables.
可用于赋值操作的右边但不能用于左边的值。右值只能读而不能写。
A value that can be used as the right-hand, but not left-hand side of an assignment. An rvalue may be read but not written.
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