[C++ Primer Plus] 第8章、函数探幽（一）程序清单——内联、引用、格式化输入输出、模板、decltype

程序清单8.1（inline内联函数）

#include<iostream>
using namespace std;

inline double square(double x) {//inline表示内联函数
    return x*x;
}

void main()
{
    double a, b, c = 13.0;
    a = square(5.0);
    b = square(4.5+7.5);
    cout << "a=" << a << ",b=" << b << endl;
    cout << "c=" << c << ",c square=" << square(c++) << endl;
    cout << "Now c=" <<c<< endl;
    system("pause");
}

程序清单8.2+8.3（引用变量）

引用变量的主要用途是作为函数的形参：引用变量做参数，函数将使用原始数据。

#include<iostream>
using namespace std;

void main()
{
    int rat = 101;
    int &rodent = rat;//将rodent的类型声明为int&，即指向int变量的引用
    cout << "rat=" <<rat<< ",rodent=" <<rodent<< endl;
    rodent++;
    cout << "Now,rat=" << rat << ",rodent=" << rodent << endl;
    cout << "rat address=" << &rat << endl;
    cout << "rodent address=" <<&rodent<< endl;

    cout << "************************************" << endl;
    int bunny = 50;
    rodent = bunny;
    cout << "rat=" << rat << ",rodent=" << rodent<<",bunny="<<bunny << endl;
    cout << "bunny address=" << &bunny << endl;
    cout << "rodent address=" << &rodent << endl;

    system("pause");
}

---

 临时变量、引用参数和const

如果实参与引用参数不匹配，仅当参数为const引用时，C++将生成临时变量。

程序清单8.6

#include<iostream>
#include<string>
using namespace std;

struct  free_throw
{
    string name;
    int made;
    int attempt;
    float percent;
};
void display(const free_throw &ft) {
    cout << "Name=" << ft.name << endl;
    cout << "Made=" << ft.made << "		";
    cout << "Attempt=" << ft.attempt<< "	";
    cout << "Percent=" << ft.percent<<endl;
}
void set(free_throw &ft) {
    if (ft.attempt != 0)
        ft.percent = 100.0f*float(ft.made) / float(ft.attempt);
    else
        ft.percent = 0;
}
free_throw & accumulate(free_throw &target,const free_throw &source) {
    target.attempt += source.attempt;
    target.made += source.made;
    set(target);
    return target;
}

void main()
{
    free_throw one = { "If Branch",13,14 };
    free_throw two = { "Andor Knott",10,16 };
    free_throw three = { "Min Max",7,9 };
    free_throw four = { "Whi Loo",5,9 };
    free_throw five = { "Long Long",6,14 };
    free_throw team = { "Throw",0,0};
    free_throw dup;

    set(one);
    display(one);
    accumulate(team, one);
    display(team);
    display(accumulate(team,two));
    accumulate(accumulate(team, three), four);
    display(team);
    dup = accumulate(team, five);//返回的team赋给dup
    cout << "Displaying team:" << endl;
    display(team);//team经过48行代码，已经发生了变化
    cout << "Displaying dup after assignment:" << endl;
    display(dup);
    set(four);
    accumulate(dup, five) = four;//four赋给dup
    cout << "Displaying dup after ill-advised assignment:" << endl;
    display(dup);

    system("pause");
}

 程序清单8.7

string：标准库允许把字符串字面值和字符字面值转换为 string 对象，当把 string 对象和字符字面值以及字符串字面值混在一条语句中使用时，必须确保加法运算符（ + ）的两侧运算对象至少有一个是 string:

#include <iostream>  
#include <string>  
using namespace std;

string version1(const string &s1, const string &s2) {
    string tmp;
    tmp = s2 + s1 + s2;
    return tmp;//返回main函数时，tmp不复存在，但tmp的内容被复制到临时存储单元
}
string & version2(string &s1, const string &s2) {//有副作用，改变了s1的内容
    s1 = s2 + s1 + s2;
    return s1;
}
string & version3(string &s1, const string &s2) {//坏设计
    string tmp;
    tmp = s2 + s1 + s2;
    return tmp;//返回main函数时，tmp内存已被释放，程序不能引用已经释放的内存
}

void main()
{
    string input, copy, result;
    cout << "Enter a string:";
    getline(cin, input);//用于字符串的输入
    copy = input;//C++中，string可以直接赋值
    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;

    cout << "Resetting original string." << endl;
    input = copy;
    result = version3(input, "@@@");//version3返回地址值时报错，因为该地址已释放
    cout << "Your string enhanced:" << result << endl;
    cout << "Your original string:" << input << endl;

    system("pause");
}

 程序清单8.8（格式化输入输出）

#include <iostream>  
#include <fstream>  
#include <cstdlib>  
using namespace std;
const int LIMIT = 5;

void file_it(ostream &os,double fo,const double fe[],int n) {//参数os可以指向ostream对象（如cout），也可以指向ofstream对象（如fout）
    ios_base::fmtflags initial;//ios_base::fmtflags是存储这种信息所需的数据类型名称
    initial = os.setf(ios_base::fixed);//setf(ios_base::fixed)使用定点表示法 表示对象
    os.precision(0);//指定显示多少位小数（假定处于定点模式下）
    os << "Focal length of objective:" << fo << " mm"<<endl;
    os.setf(ios::showpoint);//使用小数点 表示对象，即使小数部分为0
    os.precision(1);
    os.width(12);//设置下一次输出操作使用的字段宽度，只在显示下一个值时有效
    os << "f.1. eyepiece";
    os.width(15);
    os << "magnification" << endl;
    for (int i = 0; i < n; i++)
    {
        os.width(12);
        os << fe[i];
        os.width(15);
        os << int(fo / fe[i] + 0.5) << endl;
    }
    os.setf(initial);//返回调用它之前有效的所有格式化设置，即将对象回到传递给file_it()之前的状态
    //当程序将cout传递给file_it()时，cout的设置将被修改，然后被恢复；fout同样如此
}
void main()
{
    ofstream fout;
    const char *fn = "ep-data.txt";
    fout.open(fn);
    if (!fout.is_open()) {
        cout << "Can't open "<< fn <<".Bye."<< endl;
        exit(EXIT_FAILURE);
    }
    double objective;
    cout << "Enter ……：";
    cin >> objective;
    double eps[LIMIT];
    cout << "Enter XXX" << endl;
    for (int i = 0; i < LIMIT; i++)
    {
        cout << "Eyepiece #" << i + 1 << ": ";
        cin >> eps[i];
    }
    file_it(fout, objective, eps, LIMIT);//将数据写入文件ep-data.txt
    file_it(cout, objective, eps, LIMIT);//将同样的信息以相同的格式显示到屏幕上
    cout << "Done" << endl;
    system("pause");
}

以下是一些常见的控制函数的:
dec 置基数为10 相当于"%d"
hex 置基数为16 相当于"%X"
oct 置基数为8 相当于"%o"
setfill(c) 设填充字符为c
setprecision(n) 设显示小数精度为n位
setw(n) 设域宽为n个字符

setioflags(ios::fixed) 固定的浮点显示
setioflags(ios::scientific) 指数表示
setiosflags(ios::left) 左对齐
setiosflags(ios::right) 右对齐
setiosflags(ios::skipws 忽略前导空白
setiosflags(ios::uppercase) 16进制数大写输出
setiosflags(ios::lowercase) 16进制小写输出
setiosflags(ios::showpoint) 强制显示小数点
setiosflags(ios::showpos) 强制显示符号

 程序清单8.9（默认参数）

#include <iostream>   
using namespace std;

const int Size = 80;
char *left(const char *str, int n = 1);//默认n为1

void main()
{
    char sample[Size];
    cout << "Enter a string:
";
    cin.get(sample, Size);
    char *p = left(sample, 4);//调用此方法后要记得delete
    cout << p << endl;
    delete[] p;

    p = left(sample);
    cout << p << endl;
    delete[] p;
    system("pause");
}

char *left(const char *str, int n) {//内部使用了new未delete，所以在main函数调用此方法后一定要记得delete
    if (n < 0)
        n = 0;
    char *p = new char[n + 1];//new和delete成对出现
    int i;
    for (i = 0; i < n&&str[i]; i++)
        p[i] = str[i];
    while (i <= n)
        p[i++] = '';
    return p;
}

程序清单8.10（函数重载）

#include <iostream>   
using namespace std;

char *left(const char *str, int n) {//内部使用了new未delete，所以在main函数调用此方法后一定要记得delete
    if (n < 0)
        n = 0;
    char *p = new char[n + 1];//new和delete成对出现
    int i;
    for (i = 0; i < n&&str[i]; i++)
        p[i] = str[i];
    while (i <= n)
        p[i++] = '';
    return p;
}
unsigned long left(unsigned long num, unsigned ct) {
    if (ct == 0 || num == 0)
        return 0;
    unsigned digit = 1;
    unsigned long n = num;
    while (n /= 10)
        digit++;
    if (digit > ct) {
        ct = digit - ct;
        while (ct--)
            num /= 10;
        return num;
    }
    else
        return num;
}
void main()
{
    char *trip = "Hawaii!!";
    unsigned long n = 12345678;
    int i;
    char * temp;
    for (i = 1; i < 10; i++)
    {
        cout << left(n, i) << endl;
        temp = left(trip, i);
        cout << temp << endl;
        delete[] temp;
    }
    system("pause");
}

程序清单8.11+12（函数模板，模板重载）

#include <iostream>   
using namespace std;

template <typename T>    //函数模板
void swap2(T &a, T &b) { //引用        不能用swap函数名，用户自己定义的swap()函数与STL库定义的函数重载冲突
    T temp;
    temp = a;
    a = b;
    b = temp;
}
template <typename T>
void swap2(T a[], T b[],int n) {
    T temp;
    for (int i = 0; i < n; i++)
    {
        temp = a[i];
        a[i] = b[i];
        b[i] = temp;
    }
}

const int Size = 5;
template <class Te>//模板的另一种表示方式
void show(Te a[]) {
    for (int i = 0; i < Size; i++)
    {
        cout << a[i] << "	";
    }
    cout << endl;
}

void main()
{
    int i = 10, j = 20;
    cout << "i=" << i << ",j=" << j << endl;
    swap2(i, j);
    cout << "Now,i=" << i << ",j=" << j << endl;

    double x[Size] = { 1,2,3,4,5 };
    double y[] = {6,7,8,9,10};
    show(x);
    show(y);
    swap2(x, y ,Size);
    cout << "Now" << endl;
    show(x);
    show(y);

    system("pause");
}

程序清单8.13（模板显示具体化）

#include <iostream>   
using namespace std;

//模板：交换a、b值
template <typename T>
void Swap(T &a, T &b) {
    T temp = a;
    a = b;
    b = temp;
}

struct job {
    char name[40];
    double salary;
    int floor;
};
//显示具体化：交换结构体内部成员值
template <> void Swap<job>(job &j1, job &j2) {//引用
    double t1;
    int t2;
    t1 = j1.salary;
    j1.salary = j2.salary;
    j2.salary = t1;

    t2 = j1.floor;
    j1.floor = j2.floor;
    j2.floor = t2;
}

void show(job &j) {//引用
    cout << j.name << ":$"<<j.salary<<" on floor "<<j.floor<<endl;
}

void main()
{
    cout.precision(2);//指定显示多少位小数（假定处于定点模式下）：看有没有set ios::fixed，如果没有的话, 是代表2位有效数字, 不是3位小数
    cout.setf(ios::fixed,ios::floatfield);//ios::fixed设置为定点输出格式，floatfield设置输出时按浮点格式，小数点后有6位数字
    int i = 10, j = 20;
    cout << "i=" << i << ",j=" << j << endl;
    Swap(i, j);
    cout << "Now,i=" << i << ",j=" << j << endl;

    job sue = { "S Y",73000.60,7 };
    job sid = { "S T",78060.72,9 };
    show(sue);
    show(sid);
    Swap(sue, sid);
    show(sue);
    show(sid);
    system("pause");
}

程序清单8.14

#include <iostream>   
using namespace std;

template <typename T>
void show(T arr[],int n) {
    cout << "template A" << endl;
    for (int i = 0; i < n; i++)
        cout << arr[i] << ' ';
    cout << endl;
}
template <typename T>
void show(T * arr[], int n) {//指针数组:[]比*优先级高
    cout << "template B" << endl;
    for (int i = 0; i < n; i++)
        cout << *arr[i] << ' ';
    cout << endl;
}

struct debt {
    char name[40];
    double amount;
};

void main()
{
    int thing[6] = { 13,31,103,301,310,130 };
    struct debt mr_E[3] = {
        { "I W",2400.0 },
        { "U F",1300.0 },
        { "I S",1800.0 }
    };
    double *pd[3];
    for (int i = 0; i < 3; i++)
        pd[i] = &mr_E[i].amount;
    cout << "Listen:" << endl;
    show(thing, 6);
    cout << "Listen debts:" << endl;
    show(pd, 3);

    system("pause");
}

如果去掉  　template B　　，打印出来的将是地址

程序清单8.15

#include <iostream>   
using namespace std;

//返回小值
template <typename T>
T lesser(T a,T b) {
    return a < b ? a : b;
}
//先取绝对值，再返回绝对值中的小值
int lesser(int a, int b) {
    a = a < 0 ? -a : a;
    b = b < 0 ? -b : b;
    return a < b ? a : b;
}

void main()
{
    int m = 20, n = -30;
    double x = 15.5, y = 25.9;
    cout << lesser(m, n) << endl;//调用函数
    cout << lesser(x, y) << endl;//调用模板函数
    cout << lesser<>(m, n) << endl;//<>表示调用模板函数
    cout << lesser<int>(x,y)<<endl;//<int>表示 显示实例化，强转xy值

    system("pause");
}

 关键字decltype