C++ Prime Plus 学习笔记018

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第8章 函数探幽

8.1 C++ 内联函数

在C++中，内联函数是一种用于提升程序执行效率的机制。通过使用 inline 关键字，编译器会尝试将函数调用直接替换为函数体内容，从而避免函数调用带来的开销。

以下是一个展示内联函数使用的示例代码：

#include <iostream>
inline double square(double x) {return x*x;}
int main()
{
    using namespace std;
    double a, b;
    double c = 13.0;
    a = square(5.0);
    b = square(4.5 + 7.5);
    cout << "a= " << a << ", b=" << b << endl;
    cout << "c= " << c;
    cout << ", c squared = " << square(c++) << endl;
    cout << "Now c=" << c << endl;
    return 0;
}

该程序的运行输出结果如下：

a= 25, b=144
c= 13, c squared = 169
Now c=14
--------------------------------
Process exited after 0.3357 seconds with return value 0
请按任意键继续. . .

[此处为图片1]

8.2 引用变量

引用是C++中一种重要的语言特性，它允许为一个已存在的变量创建一个别名。引用必须在声明时初始化，并且一旦绑定到某个变量后就不能再更改指向。

下面的示例演示了引用的基本用法：

#include <iostream>
int main()
{
    using namespace std;
    int rats = 101;
    int & rodents = rats;
    cout << "rats= " << rats << ", rodents=" << rodents << endl;
    rodents++;
    cout << "rats= " << rats << ", rodents=" << rodents << endl;
    cout << "rats address= " << &rats << ", rodents address =" << &rodents << endl;
    return 0;
}

程序执行后的输出为：

rats= 101, rodents=101
rats= 102, rodents=102
rats address= 0x6ffe04, rodents address =0x6ffe04
--------------------------------
Process exited after 0.3505 seconds with return value 0
请按任意键继续. . .

[此处为图片2]

另一个例子进一步说明引用的行为特点。当引用被初始化后，它始终指向原始变量，即使对引用赋值也只是修改原变量的值，而非改变其引用关系。

#include <iostream>
int main()
{
    using namespace std;
    int rats = 101;
    int & rodents = rats;
    cout << "rats= " << rats << ", rodents=" << rodents << endl;
    cout << "rats address= " << &rats << ", rodents address =" << &rodents << endl;
    int bunnies = 50;
    rodents = bunnies;
    cout << "bunnies= " << bunnies << ", rats=" << rats << ", rodents= " << rodents << endl;
    cout << "bunnies address=" << &bunnies << ", rats address= " << &rats << ", rodents address =" << &rodents << endl;
    return 0;
}

对应的运行结果为：

rats= 101, rodents=101
rats address= 0x6ffdf4, rodents address =0x6ffdf4
bunnies= 50, rats=50, rodents= 50
bunnies address=0x6ffdf0, rats address= 0x6ffdf4, rodents address =0x6ffdf4
--------------------------------
Process exited after 0.3265 seconds with return value 0
请按任意键继续. . .

[此处为图片3]

环境信息：

• 书籍：C++ Primer Plus（第六版）（中文版）
• 开发工具：Dev-C++ 5.11
• CPU型号：Intel(R) Xeon(R) CPU E5-2603 v3 @ 1.60GHz
• 系统类型：64位操作系统，基于X64的处理器，Windows 10 专业版

实例8.5 cubes.cpp

#include <iostream>
double cube(double a);
double refcube(double &ra);
int main()
{
    using namespace std;
    double x = 3.0;
    cout << cube(x) << " =cube of " << x << endl;
    cout << refcube(x) << " =cube of " << x << endl;
    return 0;
}
double cube(double a)
{
    a *= a * a;
    return a;
}
double refcube(double &ra)
{
    ra *= ra * ra;
    return ra;
}

程序运行结果：

27 =cube of 3
27 =cube of 3

--------------------------------
Process exited after 0.3407 seconds with return value 0
请按任意键继续. . .

实例8.6 strtref.cpp

#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);

实例8.4 swaps.cpp

#include <iostream>
void swapr(int & a, int & b);
void swapp(int * p, int * q);
void swapv(int a, int b);
int main()
{
    using namespace std;
    int w1 = 300;
    int w2 = 350;
    cout << "w1= " << w1 << ", w2=" << w2 << endl;
    cout << "using references to swap contents:\n";
    //swapr(w1,w2);
    cout << "w1= " << w1 << ", w2=" << w2 << endl;
    cout << "using pointers to swap contents:\n";
    //swapp(&w1,&w2);
    cout << "w1= " << w1 << ", w2=" << w2 << endl;
    cout << "Trying to use passing by value:\n";
    swapv(w1, w2);
    cout << "w1= " << w1 << ", w2=" << w2 << endl;
    return 0;
}
void swapr(int &a, int &b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}
void swapp(int *p, int *q)
{
    int temp;
    temp = *p;
    *p = *q;
    *q = temp;
}
void swapv(int a, int b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}

执行输出结果：

w1= 300, w2=350
using references to swap contents:
w1= 300, w2=350
using pointers to swap contents:
w1= 300, w2=350
Trying to use passing by value:
w1= 300, w2=350

--------------------------------
Process exited after 0.3774 seconds with return value 0
请按任意键继续. . .

bunnies address=0x6ffdf0, rats address= 0x6ffdf4, rodents address =0x6ffdf4

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

struct free_throws {
    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 = {"Branch", 13, 14};
    free_throws two = {"Knot", 10, 16};
    free_throws three = {"Max", 7, 9};
    free_throws four = {"Looper", 5, 9};
    free_throws five = {"Long", 6, 14};
    free_throws team = {"Goods", 0, 0};
    free_throws dup;

    set_pc(one);
    display(one);
    accumulate(team, one);
    display(team);
    display(accumulate(team, two));
    accumulate(accumulate(team, three), four);
    display(team);

    dup = accumulate(team, five);
    std::cout << "Display team:\n";
    display(team);

    std::cout << "Displaying dup after assignment:\n";
    display(dup);

    set_pc(four);
    accumulate(dup, five) = four;
    std::cout << "Displaying dupp 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 = 100.0 * 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;
}

运行输出结果如下所示：

name: Branch
 made: 13     Attempts: 14     percent: 92.8571
name: Goods
 made: 13     Attempts: 14     percent: 92.8571
name: Goods
 made: 23     Attempts: 30     percent: 76.6667
name: Goods
 made: 35     Attempts: 48     percent: 72.9167
Display team:
name: Goods
 made: 41     Attempts: 62     percent: 66.129
Displaying dup after assignment:
name: Goods
 made: 41     Attempts: 62     percent: 66.129
Displaying dupp after ill-advised assignment:
name: Looper
 made: 5      Attempts: 9      percent: 55.5556
--------------------------------
Process exited after 0.3639 seconds with return value 0
请按任意键继续. . .

[此处为图片1] 该程序演示了结构体与函数引用的结合使用，重点在于通过引用传递来高效地修改和累积数据。accumulate 函数接收两个参数：目标对象和源对象，并将源对象的投篮次数和命中数累加到目标对象中，同时更新命中率。display 函数用于格式化输出每个球员的数据信息。set_pc 函数则负责计算并设置命中百分比。 在主函数中，首先初始化多个 free_throws 类型的变量，代表不同球员的罚球统计。随后调用 set_pc 和 accumulate 等函数进行数据处理，并逐步输出中间结果。最后展示了对引用返回值进行错误赋值所导致的异常行为，强调了合理使用引用的重要性。

#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);

int main()
{
    ofstream fout;
    const char *fn = "ep-data.txt";
    fout.open(fn);
    
    if (!fout.is_open())
    {
        cout << "Can't open " << fn << ". Bye.\n";
        exit(EXIT_FAILURE);
    }
    
    double objective;

该程序的主要功能是向指定文件写入格式化数据。通过定义一个输出流参数，函数 file_it 能够灵活地将信息输出到不同目标，例如文件或标准输出设备。

string version1(const string &s1, const string &s2);
const string &version2(string &s1, const string &s2);
const string &version3(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;

    cout << "Resetting original string:\n";
    input = copy;

    result = version3(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)
{
    s1 = s2 + s1 + s2;
    return s1;
}

const string &version3(string &s1, const string &s2)
{
    string temp;
    temp = s2 + s1 + s2;
    return temp;
}

运行输出结果如下所示：

Enter a string: It's not my fault.
Your string as entered: It's not my fault.
Your string enhanced: ***It's not my fault.***
Your original string: It's not my fault.
Your string enhanced: ###It's not my fault.###
Your original string: ###It's not my fault.###
Resetting original string:

[此处为图片1]

--------------------------------
Process exited after 18.19 seconds with return value 3221226356
请按任意键继续. . .

示例代码 filefunc.cpp 展示了如何使用文件流进行数据存储操作。其中，函数 file_it 接收一个通用的输出流引用，使得它既能输出到文件，也可用于屏幕显示，提高了代码复用性。

#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

const int LIMIT = 5;

void file_it(ostream &os, double fo, const double fe[], int n);

int main()
{
    double objective;
    cout << "Enter the focal length of your telescope objective in mm: ";
    cin >> objective;

    double eps[LIMIT];
    cout << "Enter the focal lengths, in mm, of " << LIMIT << " eyepieces:\n";
    
    for (int i = 0; i < LIMIT; i++)
    {
        cout << "Eyepiece #" << i + 1 << ": ";
        cin >> eps[i];
    }

    ofstream fout("output.txt");
    if (fout.is_open())
    {
        file_it(fout, objective, eps, LIMIT);
        fout.close();
    }

    file_it(cout, objective, eps, LIMIT);
    cout << "Done.\n";
    return 0;
}

void file_it(ostream &os, double fo, const double fe[], int n)
{
    ios_base::fmtflags initial;
    initial = os.setf(ios_base::fixed);
    os.precision(0);
    os << "Focal length of objective: " << fo << "mm\n";

    os.setf(ios::showpoint);
    os.precision(1);

    os.width(12);
    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);
}

程序执行输出示例：

Enter the focal length of your telescope objective in mm: 1800
Enter the focal lengths, in mm, of 5 eyepieces:
Eyepiece #1: 30
Eyepiece #2: 19
Eyepiece #3: 14
Eyepiece #4: 8.9
Eyepiece #5: 7.5
Focal length of objective: 1800mm
f.1. eyepiece    magnification
        30.0             60
        19.0             95
        14.0            129
         8.9            202
         7.5            240
Done.

[此处为图片1] 程序运行结束，返回值为 0。 执行耗时约 25.58 秒。 请按任意键继续...

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