第85课 双双指向
双双指向（指针、指针与字符串、指针与函数）
函数传参通常使用值传递，即把变量的值复制给函数内部的新变量，这样在函数内对新变量的任何更改都不会影响原始变量的值。然而，有一种方法可以让函数改变其外部变量的值，这种方法就是引用传递。
引用传递
运行程序：
引用传递就是传递参数的引用，实际上是将参数变量取了一个别名传递给函数，它访问的还是原变量的内存地址，所以函数内部对参数的修改会直接影响函数外部的变量。
敲黑板
引用变量的定义方法和常规变量类似，区别在于数据类型和名称之间多了一个&符号。
例如：char &ch1
实际变量a，引用变量ch1。
引用变量ch1就好像变量a的别名，它们共同指向同一个内存地址，所以操作ch1和操作a的结果是一样的，都操作了内存里的'A'。
对比学习
通常函数是改变不了主函数中的变量的，但是通过引用变量和传地址的方式，都可以实现原变量的操作。
运行程序：
巩固练习
（1）引用传递与指针传递相比，主要的区别在于（）。
A．引用传递需要在函数内部重新创建一个变量
B．引用传递中函数()里的参数int& ch表示变量ch的地址
C．引用传递不需要额外的内存分配
D．当函数中的参数为指针变量int*p时，可以直接传入一个整数变量
（2）阅读以下程序，填写程序的输出结果。
输出：_____________________________________________________________ _____
（3）阅读以下程序，填写程序的输出结果。
输出：_____________________________________________________________ _____
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