引用变量

被 palabos 倒逼学 C++。

1 引用变量

#include <iostream>
int main()
{
    using namespace std;
    int rats = 101;
    int & rodents = rats;   // 一个 int 类型的引用

    cout << "rats = " << rats;
    cout << ", rodents = " << rodents << endl;
    rodents++;
    cout << "rats = " << rats;
    cout << ", rodents = " << rodents << endl;

    cout << "rats address = " << &rats;
    cout << ", rodents address = " << &rodents << endl;
    // 两个地址的值是一样的
    return 0; 
}

rats 和 rodents 指向相同的值和内存单元。引用创建的时候就得赋值。引用应该只和一个变量相关联，不然会有意想不到的问题：

// secref.cpp -- defining and using a reference
#include <iostream>
int main()
{
    using namespace std;
    int rats = 101;
    int & rodents = rats;   

    cout << "rats = " << rats;
    cout << ", rodents = " << rodents << endl;

    cout << "rats address = " << &rats;
    cout << ", rodents address = " << &rodents << endl;

    int bunnies = 50;
    rodents = bunnies;      
    cout << "bunnies = " << bunnies;
    cout << ", rats = " << rats;
    cout << ", rodents = " << rodents << endl;

    cout << "bunnies address = " << &bunnies;
    cout << ", rodents address = " << &rodents << endl;
    // cin.get();
    return 0; 
}

输出结果：

rats = 101, rodents = 101
rats address = 0x61fe14, rodents address = 0x61fe14
bunnies = 50, rats = 50, rodents = 50
bunnies address = 0x61fe10, rodents address = 0x61fe14

1-1 引用用作函数参数

引用作为函数的参数，使得函数中的变量名成为程序中变量名的别名。函数可以访问调用函数中的变量。

常引用

如果想让函数使用传递给它的信息，有不能修改信息，可以使用 常引用。这个时候编译器发现函数试图修改信息，就会生成错误消息。

#include <iostream>
double cube(double a);
double refcube(double const &ra); // const 关键字放在 “类型名” 前面后面都可以
int main ()
{
    using namespace std;
    double x = 3.0;

    cout << cube(x);
    cout << " = cube of " << x << endl;
    cout << " = cube of " << refcube(x) << endl;
    // cin.get();
    return 0;
}

double cube(double a)
{
    a *= a * a;
    return a;
}

double refcube(const double &ra)
{
    int a;
    a = ra * ra * ra;
    return a; 
}

1-2 引用用于结构 和 类对象

是为了处理 “结构” 和 “类” 这些用户定义的类型。

函数返回引用

返回一个作为参数传递给函数的引用。作为参数的引用指向函数使用的数据，所以返回的引用也指向这些数据。

//strc_ref.cpp -- using structure references
#include <iostream>
#include <string>
struct free_throws
{
    std::string name;
    int made;
    int attempts;
    float percent;
};

void display(const free_throws & ft);
void set_pc(free_throws & ft);
free_throws & accumulate(free_throws &target, const free_throws &source);

int main()
{
    free_throws one = {"Ifelsa Branch", 13, 14};
    free_throws two = {"Andor Knott", 10, 16};
    free_throws three = {"Minnie Max", 7, 9};
    free_throws four = {"Whily Looper", 5, 9};
    free_throws five = {"Long Long", 6, 14};
    free_throws team = {"Throwgoods", 0, 0};
    free_throws dup;
    set_pc(one);
    display(one);
    accumulate(team, one);
    display(team);
// 返回一个作为参数传递给函数的引用
    display(accumulate(team, two));
    accumulate(accumulate(team, three), four);
    display(team);
// 返回一个作为参数传递给函数的引用
    dup = accumulate(team,five); // 结构直接赋值了
    std::cout << "Displaying team:\n";
    display(team);
    std::cout << "Displaying dup after assignment:\n";
    display(dup);
    set_pc(four);
// ill-advised assignment
    accumulate(dup,five) = four; // 左边返回的 dup 被修改了
    std::cout << "Displaying dup after ill-advised assignment:\n";
    display(dup); 
    return 0;
}

void display(const free_throws & ft)
{
    using std::cout;
    cout << "Name: " << ft.name << '\n';
    cout << "  Made: " << ft.made << '\t';
    cout << "Attempts: " << ft.attempts << '\t';
    cout << "Percent: " << ft.percent << '\n';
}
void set_pc(free_throws & ft)
{
    if (ft.attempts != 0)
        ft.percent = 100.0f *float(ft.made)/float(ft.attempts);
    else
        ft.percent = 0;
}

free_throws & accumulate(free_throws & target, const free_throws & source)
{
    target.attempts += source.attempts;
    target.made += source.made;
    set_pc(target);
    return target;
}

引用用于类对象

#include <iostream>
#include <string>
using namespace std;
string version1(const string & s1, const string & s2);
const string & version2(string & s1, const string & s2);  
 
int main()
{
    string input;
    string copy;
    string result;

    cout << "Enter a string: ";
    getline(cin, input);
    copy = input;
    cout << "Your string as entered: " << input << endl;
    result = version1(input, "***");
    cout << "Your string enhanced: " << result << endl;
    cout << "Your original string: " << input << endl;

    result = version2(input, "###");
    cout << "Your string enhanced: " << result << endl;
    cout << "Your original string: " << input << endl;

    return 0;
}

string version1(const string & s1, const string & s2)
{
    string temp;

    temp = s2 + s1 + s2;
    return temp;
}

const string & version2(string & s1, const string & s2)   // has side effect
{
    s1 = s2 + s1 + s2;
    return s1; 
}

不能返回一个作用域在函数中，函数结束，它就自动销毁的对象。