下面是我阅读《 C++ Primer 》中“Argument Passing”章节做的一个小结，，大家看看有没有错误

参数

passed by value

the value of the argument is copied to the parameter, nothing the function does to the parameter can affect the argument

passed by pointer

behave like passed by value, the value of the argument pointer is copied to the parameter pointer, the two pointers are distinct. However, a pointer also gives us indirect access to the object to which that pointer points.

修改 argument pointer 本身不会影响 parameter pointer，但通过 argument pointer 修改它指向的对象，会同样修改 parameter pointer 指向的对象，因为它们指向同一个对象。

passed by reference

operations on a reference are actually operations on the object to which the reference refers, often used to allow a function to change the value of its arguments

引用传参有两个作用：

• 避免拷贝
• 修改被引用对象的值，实现多返回值效果

对于原始类型参数，拷贝代价很小，只有当需要利用它返回额外信息时才需要引用传参

对于复合类型参数，引用传参一般是为了避免拷贝，不会在函数内部给它重新赋值，因此一般会用const修饰