C++基础 （5章）知识点：引用

引用

引用是C++对C的扩充，通过变量名来申请并命名内存空间。 重点概念： （1）引用的基本使用方法 （2）引用使用时需要注意的三件事 （3）引用数组 （4）三种传递方法 （5）引用作为返回值 （6）引用指针 （7）引用常量

5.1引用的基本使用方法

Type &ref = val；

注意： （1）&在此式中不是求址符，而是起到标识作用 （2）必须在声明引用变量时进行初始化 （3）引用初始化之后不能改变 （4）不能有NULL引用，必须确保引用是和一块合法的存储单元关联 （5）可以建立对数组的引用

void referenced_test_5_1() { int a = 10; int &b = a; cout << "a" <<a << endl; cout << "b" << b << endl; cout << "------" << endl; b = 100; cout << "a" << a << endl; cout << "b" << b << endl; cout << "------" << endl; int &c = a; int &d = b; cout << "a" << a << endl; cout << "b" << b << endl; cout << "c" << c << endl; cout << "d" << d << endl; cout << "------" << endl; }

5.2引用的使用要注意哪些

（1）引用必须初始化 （2）引用初始化之后其指向的地址不能改变 （3）不能直接建立引用数组

void referenced_test_5_2() { //（1）引用必须初始化 //int& ref; //（2）引用初始化之后不能进行改变 int a, b; int &ref = a; //&ref = b; //（3）不能直接对数组建立引用 //int c[20]; //int &reff = c; //int &reff[20] = c; }

5.3引用数组

void referenced_test_5_3() { typedef int ArrRef[10]; int arr[10]; ArrRef &ref = arr; for (int i = 0; i < 10; i++) { arr[i] = i; cout << "ref[i]: " << ref[i] << endl; } cout << endl; int(&f)[10] = arr; for (int i = 0; i < 10; i++) { cout << "f[i]: " << f[i] << endl; } }

5.4三种传递方式

（1）值传递

void ValueSwap(int a, int b) { int temp; temp = b; b = a; a = temp; }

（2）地址传递

void PointerSwap(int *a, int *b) { int temp; temp = *b; *b = *a; *a = temp; }

（3）引用传递

void ReferencedSwap(int &a, int &b) { int temp; temp = b; b = a; a = temp; } void referenced_test_5_4() { int a = 5; int b = 10; ValueSwap(a, b); cout << "a:" << a << " b:" << b << endl; a = 5; b = 10; PointerSwap(&a, &b); cout << "a:" << a << " b:" << b << endl; a = 5; b = 10; ReferencedSwap(a, b); cout << "a:" << a << " b:" << b << endl; }

结果分析：

a:5 b:10 a:10 b:5 a:10 b:5

在地址传递和引用传递时，形参发生改变变量的结果也会发生改变

5.5引用作为返回值

（1）返回局部变量，方法不可取，地址会被释放，值容易丢失

int& referenced_test_5_5_1() { int a = 30; return a; }

（2）返回静态变量引用

int& referenced_test_5_5_2() { static int a = 10; return a; } void referenced_test_5_5() { cout << "局部变量引用返回" << "a:" << referenced_test_5_5_1() << endl; cout << "局部静态变量引用返回" << "a:" << referenced_test_5_5_2() << endl; }

5.6指针引用

c语言中如果想改变一个指针的指向而不是它所指向的内容，函数声明一般为：

void fun(int** s)

而在c++中可以采取指针引用的方法改变指针所指向的内容：

void fun(int*& s)

下面是例子：

//指针间接修改teacher的年龄 void referenced_test_5_6_1(Teacher** teacher) { *teacher = (Teacher*)malloc(sizeof(Teacher)); (*teacher)->mAge = 300; } //用引用修改teacher的年龄 void referenced_test_5_6_2(Teacher*& teacher) { //teacher = (Teacher*)malloc(sizeof(Teacher)); teacher->mAge = 500; } void referenced_test_5_6() { Teacher* teacher = NULL; referenced_test_5_6_1(&teacher); cout << "5.6.1" << "teacher->mAge:" << teacher->mAge << endl; referenced_test_5_6_2(teacher); cout << "5.6.2" << "teacher->mAge:" << teacher->mAge << endl; }

5.7常量引用

const int& ref = 500； //常量指针可直接被常数赋值 //int & ref = 500； //不可用引用不可直接被常数赋值

常量引用指向自变量时，可通过改变自变量而改变常量引用

void referenced_test_5_7_1() { int a = 1000; const int& aRef = a; //aRef = 200; 编译不通过 a = 300; //编译通过 cout << "5.7.1" << endl; cout << "a:" << a << endl; cout << "aRef:" << aRef << endl; }

本章所有代码：

#include <iostream> using namespace std; //5.1 认识引用 void referenced_test_5_1() { int a = 10; int &b = a; cout << "a" <<a << endl; cout << "b" << b << endl; cout << "------" << endl; b = 100; cout << "a" << a << endl; cout << "b" << b << endl; cout << "------" << endl; int &c = a; int &d = b; cout << "a" << a << endl; cout << "b" << b << endl; cout << "c" << c << endl; cout << "d" << d << endl; cout << "------" << endl; } //引用的使用要注意哪些 void referenced_test_5_2() { //（1）引用必须初始化 //int& ref; //（2）引用初始化之后不能进行改变 int a, b; int &ref = a; //&ref = b; //（3）不能直接对数组建立引用 //int c[20]; //int &reff = c; //int &reff[20] = c; } //建立数组引用的方法 void referenced_test_5_3() { typedef int ArrRef[10]; int arr[10]; ArrRef &ref = arr; for (int i = 0; i < 10; i++) { arr[i] = i; cout << "ref[i]: " << ref[i] << endl; } cout << endl; int(&f)[10] = arr; for (int i = 0; i < 10; i++) { cout << "f[i]: " << f[i] << endl; } } //5.4 三种传递方式 //（1）值传递 void ValueSwap(int a, int b) { int temp; temp = b; b = a; a = temp; } //（2）地址传递 void PointerSwap(int *a, int *b) { int temp; temp = *b; *b = *a; *a = temp; } //（3）引用传递 void ReferencedSwap(int &a, int &b) { int temp; temp = b; b = a; a = temp; } void referenced_test_5_4() { int a = 5; int b = 10; ValueSwap(a, b); cout << "a:" << a << " b:" << b << endl; a = 5; b = 10; PointerSwap(&a, &b); cout << "a:" << a << " b:" << b << endl; a = 5; b = 10; ReferencedSwap(a, b); cout << "a:" << a << " b:" << b << endl; } //5.5引用作为函数返回值 需要注意什么 //（1）返回局部变量，方法不可取，值会丢失 int& referenced_test_5_5_1() { int a = 30; return a; } //（2）返回静态变量引用 int& referenced_test_5_5_2() { static int a = 10; return a; } void referenced_test_5_5() { cout << "局部变量引用返回" << "a:" << referenced_test_5_5_1() << endl; cout << "局部静态变量引用返回" << "a:" << referenced_test_5_5_2() << endl; } //5.6 指针引用 //c语言中如果想改变一个指针的指向而不是它所指向的内容，函数声明一般为： //void fun(**) //给指针变量取一个别名 int * pointer = NULL; //int*& = pointer; //改变指针的指向 struct Teacher { int mAge; }; //指针间接修改teacher的年龄 void referenced_test_5_6_1(Teacher** teacher) { *teacher = (Teacher*)malloc(sizeof(Teacher)); (*teacher)->mAge = 300; } //用引用修改teacher的年龄 void referenced_test_5_6_2(Teacher*& teacher) { //teacher = (Teacher*)malloc(sizeof(Teacher)); teacher->mAge = 500; } void referenced_test_5_6() { Teacher* teacher = NULL; referenced_test_5_6_1(&teacher); cout << "5.6.1" << "teacher->mAge:" << teacher->mAge << endl; referenced_test_5_6_2(teacher); cout << "5.6.2" << "teacher->mAge:" << teacher->mAge << endl; } //5.7常量引用 const int& ref = 500; void referenced_test_5_7_1() { int a = 1000; const int& aRef = a; //aRef = 200; 编译不通过 a = 300; //编译通过 cout << "5.7.1" << endl; cout << "a:" << a << endl; cout << "aRef:" << aRef << endl; } void referenced_test_5_7_2() { //不能把一个自变量赋给引用，但是可以赋给常量引用 //int& ref = 100; const int& ref = 100; } void referenced_test_5_7() { referenced_test_5_7_1(); referenced_test_5_7_2(); } void main() { referenced_test_5_1(); //引用的基本使用方法 referenced_test_5_2(); //引用使用时需要注意的三件事 referenced_test_5_3(); //引用数组 referenced_test_5_4(); //三种传递方法 referenced_test_5_5(); //引用作为返回值需要注意什么 referenced_test_5_6(); //引用指针 referenced_test_5_7(); //引用常量 while (1); }