We execute a member function in much the same way as we do any function: To call a function, we follow the function name by the call operator (the "()" operator).
执行成员函数和执行其他函数相似：要调用函数，可将调用操作符（（））放在函数名之后。
When we use a member function as the right-hand operand of the dot operator, we usually do so to call that function.
通常使用成员函数作为点操作符的右操作数来调用成员函数。
The call operator is a pair of parentheses that encloses a (possibly empty) list of arguments that we pass to the function.
调用操作符是一对圆括号，括住传递给函数的实参列表（可能为空）。
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