• 《C++PrimePlus》第8章 函数探幽

    8.1 内联函数

    使用内联函数

    1. #include 
    2. using namespace std;
    3.  
    4. inline double square(double x) { return x * x; }
    5.  
    6. int main(){
    7. 	double a;
    8. 	a = square(5.0);
    9. 	cout << "a = " << a << endl;
    10. 	return 0;
    11. }

    8.2 引用变量

    将引用用作函数参数（使用const）

    1. #include 
    2. using namespace std;
    3. double cube(const double &ra);
    4.  
    5. int main(){
    6. 	double x = 3.0;
    7. 	cout << cube(x) << " = cube of " << x << endl;
    8. 	cout << cube(5) << " = cube of " << "5" << endl;
    9. 	cout << cube(x+5) << " = cube of " << x+5 << endl;
    10. 	return 0;
    11. }
    12.  
    13. double cube(const double &ra) {
    14. 	return ra*ra*ra;
    15. }

    将引用用于结构

    1. #include 
    2. #include 
    3. using namespace std;
    4.  
    5. struct free_throws {
    6. 	string name;
    7. 	int made;
    8. 	int attempts;
    9. 	float percent;
    10. };
    11.  
    12. void set_pc(free_throws &ft);
    13. void display(const free_throws &ft);
    14. free_throws &accumulate(free_throws &target, const free_throws &source);
    15.  
    16. int main(){
    17. 	free_throws one = { "Rick", 13, 14 }; // 最后一个值没赋值，为空
    18. 	free_throws two = { "Jack", 10, 16 };
    19. 	free_throws team = { "All", 0, 0 };
    20. 	set_pc(one); // 赋值
    21. 	display(one); // 展示
    22. 	display(accumulate(team, one)); // 汇总
    23. 	return 0;
    24. }
    25.  
    26. void set_pc(free_throws &ft) { // 要修改原始数据，不加const
    27. 	if (ft.attempts != 0)
    28. 		ft.percent = 100.0 * float(ft.made) / float(ft.attempts);
    29. 	else
    30. 		ft.attempts = 0;
    31. }
    32.  
    33. void display(const free_throws &ft) {
    34. 	cout << "Name: " << ft.name << endl;
    35. 	cout << "Made: " << ft.made << '\t';
    36. 	cout << "Attempts: " << ft.attempts << '\t';
    37. 	cout << "Percent: " << ft.percent << endl;
    38. }
    39. // 把函数的返回值定义为结构体引用
    40. free_throws &accumulate(free_throws &target, const free_throws &source) {
    41. 	target.attempts += source.attempts;
    42. 	target.made += source.made;
    43. 	set_pc(target);
    44. 	return target;
    45. }

    将引用用于类的对象

    1. #include 
    2. #include 
    3. using namespace std;
    4. string version1(const string &s1, const string &s2);
    5. const string &version2(string &s1, const string &s2);
    6. // const string &version3(string &s1, const string &s2);
    7.  
    8. int main(){
    9. 	string input;
    10. 	string copy;
    11. 	string result;
    12. 	cout << "Enter a string: ";
    13. 	getline(cin, input);
    14. 	copy = input;
    15. 	cout << "You string: " << input << endl;
    16. 	result = version1(input, "***"); // 在字符串前后都加上***
    17. 	cout << "Your string enhanced: " << result << endl;
    18. 	cout << "Your input: " << input << endl;
    19. 	cout << "-------------------------------------" << endl;
    20. 	result = version2(input, "###");
    21. 	cout << "Your string enhanced: " << result << endl;
    22. 	cout << "Your input: " << input << endl;
    23. 	//cout << "-------------------------------------" << endl;
    24. 	//input = copy;
    25. 	//result = version3(input, "@@@");
    26. 	//cout << "Your string enhanced: " << result << endl;
    27. 	//cout << "Your input: " << input << endl;
    28. 	return 0;
    29. }
    30. // const string &s2 对应的是 "***"
    31. // 当使用const限定符时，会产生临时变量并进行类型转换
    32. string version1(const string &s1, const string &s2) {
    33. 	string temp;
    34. 	temp = s2 + s1 + s2;
    35. 	return temp;
    36. }
    37. // 返回一个string类的对象的引用
    38. const string &version2(string &s1, const string &s2) {
    39. 	s1 = s2 + s1 + s2;
    40. 	return s1;
    41. }
    42.  
    43. /*错误的使用方法：返回临时变量的引用
    44. const string &version3(string &s1, const string &s2) {
    45. 	string temp;
    46. 	temp = s2 + s1 + s2;
    47. 	return temp;
    48. }
    49. */

    对象、继承和引用

    1. #include 
    2. #include 
    3. #include 
    4. #include 
    5. using namespace std;
    6. const int LIMIT = 5;
    7. void file_it(ostream &os, double fo, const double fe[], int n);
    8.  
    9. int main(){
    10. 	fstream fout;
    11. 	// 先在路径中里新建这个txt文件
    12. 	const char *fn = "ep-data.txt";
    13. 	fout.open(fn);
    14. 	if (!fout.is_open()) {
    15. 		cout << "Can't open " << fn << "." << endl;
    16. 		exit(EXIT_FAILURE);
    17. 	}
    18.  
    19. 	double objective; // 物镜的焦距
    20. 	cout << "Enter the focal length of telescope objective in mm: ";
    21. 	cin >> objective;
    22. 	double eps[LIMIT]; // 目镜的焦距
    23. 	for (int i = 0; i < LIMIT; i++) {
    24. 		cout << "Eyepieces #" << i + 1 << ": ";
    25. 		cin >> eps[i];
    26. 	}
    27.  
    28. 	file_it(cout, objective, eps, LIMIT); // 在终端上显示
    29. 	file_it(fout, objective, eps, LIMIT); // 在文件中显示
    30. 	cout << "Done." << endl;
    31. 	return 0;
    32. }
    33. // ostream &os 基类的引用，可以指向基类的对象，也可以指向派生类的对象
    34. void file_it(ostream &os, double fo, const double fe[], int n) {
    35. 	os << "Focal length of objective: " << fo << endl;
    36. 	os << "f.l. eyepieces" << " magnification" << endl;
    37. 	for (int i = 0; i < n; i++) {
    38. 		os << "    " << fe[i] << "    " << int(fo / fe[i] + 0.5) << endl;
    39. 	}
    40. }

    8.3 默认参数

    默认参数的用法（取出字符串的前n个值）

    1. #include 
    2. using namespace std;
    3. const int ArSize = 80;
    4. char *left(const char *str, int n = 1); // 默认参数n=1
    5.  
    6. int main(){
    7. 	char sample[ArSize];
    8. 	cout << "Enter a string: " << endl;
    9. 	cin.get(sample, ArSize);
    10.  
    11. 	char *ps = left(sample, 4);
    12. 	cout << ps << endl;
    13. 	delete[] ps; // 注意new和delete成对出现
    14. 	ps = left(sample); // 使用默认参数
    15. 	cout << ps << endl;
    16. 	delete[] ps;
    17.  
    18. 	return 0;
    19. }
    20.  
    21. char *left(const char *str, int n) {
    22. 	int m = 0;
    23. 	while (m < n && str[m] != '\0') m++; // 确定字符串长度
    24. 	char *p = new char[m + 1];
    25. 	int i;
    26. 	for (i = 0; i < m; i++) {
    27. 		p[i] = str[i];
    28. 	}
    29. 	p[i] = '\0'; // 最后要补上一个空字符
    30. 	return p;
    31. }

    8.4 函数重载

    函数重载示例（取出字符串/数字的前n个值）

    1. #include 
    2. using namespace std;
    3. const int ArSize = 80;
    4. char *left(const char *str, int n = 1);
    5. unsigned long left(unsigned long num, unsigned int ct);
    6.  
    7. int main(){
    8. 	const char *trip = "Hawaii";
    9. 	unsigned long n = 12345678;
    10. 	int i;
    11. 	char *temp;
    12. 	for (i = 0; i < 10; i++) {
    13. 		cout << left(n, i) << endl;
    14. 		temp = left(trip, i);
    15. 		cout << temp << endl;
    16. 		delete[] temp;
    17. 	}
    18. 	return 0;
    19. }
    20.  
    21. char *left(const char *str, int n) {
    22. 	int m = 0;
    23. 	while (m < n && str[m] != '\0') m++; // 确定字符串长度
    24. 	char *p = new char[m + 1];
    25. 	int i;
    26. 	for (i = 0; i < m; i++) {
    27. 		p[i] = str[i];
    28. 	}
    29. 	p[i] = '\0'; // 最后要补上一个空字符
    30. 	return p;
    31. }
    32.  
    33. unsigned long left(unsigned long num, unsigned int ct) {
    34. 	unsigned long n = num;
    35. 	unsigned int digits = 1;
    36. 	if (num == 0 || ct == 0) return 0; // 特殊情况
    37. 	while (n /= 10) digits++; // 判断数字有几位
    38. 	if (digits > ct) {
    39. 		ct = digits - ct; // 要除几次10
    40. 		while (ct--) num /= 10;
    41. 		return num;
    42. 	}
    43. 	else
    44. 		return num;
    45. }

    8.5 函数模板

    函数模板示例（交换两个数的值）

    1. #include 
    2. using namespace std;
    3. template <typename T>
    4. void Swap(T &a, T &b);
    5.  
    6. int main(){
    7. 	int i = 10;
    8. 	int j = 20;
    9.  
    10. 	cout << "i, j = " << i << ", " << j << "." << endl;
    11. 	Swap(i, j);
    12. 	cout << "Afer swap, i, j = " << i << ", " << j << "." << endl;
    13. 	
    14. 	double x = 24.5;
    15. 	double y = 81.7;
    16. 	cout << "x, y = " << x << ", " << y << "." << endl;
    17. 	Swap(x, y);
    18. 	cout << "Afer swap, x, y = " << x << ", " << y << "." << endl;
    19.  
    20. 	return 0;
    21. }
    22.  
    23. template <typename T>
    24. void Swap(T &a, T &b) {
    25. 	T temp;
    26. 	temp = a;
    27. 	a = b;
    28. 	b = temp;
    29. }

    重载的模板示例（交换两个数或两个数组）

    1. #include 
    2. using namespace std;
    3. template <typename T>
    4. void Swap(T &a, T &b);
    5. template <typename T>
    6. void Swap(T a[], T b[], int n);
    7. const int LIMIT = 8;
    8. void show(int arr[], int n);
    9.  
    10.  
    11. int main(){
    12. 	int i = 10;
    13. 	int j = 20;
    14. 	cout << "i, j = " << i << ", " << j << "." << endl;
    15. 	Swap(i, j);
    16. 	cout << "Afer swap, i, j = " << i << ", " << j << "." << endl;
    17. 	
    18. 	int d1[LIMIT] = { 0,7,0,4,1,7,7,6 };
    19. 	int d2[LIMIT] = { 0,7,2,0,1,9,6,9 };
    20. 	cout << "Original arrays: " << endl;
    21. 	show(d1, LIMIT);
    22. 	show(d2, LIMIT);
    23. 	Swap(d1, d2, LIMIT);
    24. 	cout << "After swap: " << endl;
    25. 	show(d1, LIMIT);
    26. 	show(d2, LIMIT);
    27.  
    28. 	return 0;
    29. }
    30.  
    31. template <typename T>
    32. void Swap(T &a, T &b) {
    33. 	T temp;
    34. 	temp = a;
    35. 	a = b;
    36. 	b = temp;
    37. }
    38.  
    39. template <typename T>
    40. void Swap(T a[], T b[], int n) {
    41. 	T temp;
    42. 	for (int i = 0; i < n; i++) {
    43. 		temp = a[i];
    44. 		a[i] = b[i];
    45. 		b[i] = temp;
    46. 	}
    47. }
    48.  
    49. void show(int arr[], int n) {
    50. 	for (int i = 0; i < n; i++) {
    51. 		cout << arr[i] << " ";
    52. 	}
    53. 	cout << endl;
    54. }

    调用函数时的最佳匹配（打印数组内容）

    1. #include 
    2. using namespace std;
    3. template <typename T>
    4. void ShowArray(T arr[], int n);
    5. template <typename T>
    6. void ShowArray(T *arr[], int n);
    7.  
    8. struct debts {
    9. 	char name[50]; // 名字
    10. 	double amount; // 数量
    11. };
    12.  
    13. int main(){
    14. 	int things[6] = { 13,31,103,301,310,130 };
    15. 	struct debts mr_E[3] = 
    16. 	{
    17. 		{"Rick", 2400.00},
    18. 		{"Jack", 1300.0},
    19. 		{"Rose", 1800.0}
    20. 	};
    21. 	double *pd[3]; // 3个元素的数组，每个元素都是指针
    22. 	for (int i = 0; i < 3; i++) {
    23. 		pd[i] = &mr_E[i].amount;
    24. 	}
    25. 	ShowArray(things, 6);
    26. 	// 更匹配 void ShowArray(T *arr[], int n)
    27. 	// 会打印出来指针指向的数值
    28. 	ShowArray(pd, 3); 
    29. 	return 0;
    30. }
    31.  
    32. template <typename T>
    33. void ShowArray(T arr[], int n) {
    34. 	cout << "template A:" << endl;
    35. 	for (int i = 0; i < n; i++)
    36. 		cout << arr[i] << " ";
    37. 	cout << endl;
    38. }
    39.  
    40. template <typename T>
    41. void ShowArray(T *arr[], int n) {
    42. 	cout << "template B:" << endl;
    43. 	for (int i = 0; i < n; i++) 
    44. 		cout << *arr[i] << " ";
    45. 	cout << endl;
    46. }

    引导编译器使用指定函数（打印较小的值）

    1. #include 
    2. using namespace std;
    3. template <class T>
    4.  
    5. T lesser(T a, T b) { // 函数1 返回较小值
    6. 	return a < b ? a : b;
    7. }
    8.  
    9. int lesser(int a, int b) { // 函数2 返回绝对值的较小值
    10. 	a = a < 0 ? -a : a;
    11. 	b = b < 0 ? -b : b;
    12. 	return a < b ? a : b;
    13. }
    14.  
    15. int main(){
    16. 	int m = 20, n = -30;
    17. 	double x = 15.5, y = -25.9;
    18. 	// 非模板函数优先，调用的是函数2
    19. 	cout << lesser(m, n) << endl;
    20. 	// 非模板函数不是最优（要进行类型转换），调用的是函数1
    21. 	cout << lesser(x, y) << endl;
    22. 	// 尖括号<>告诉编译器使用模板函数，调用函数1
    23. 	cout << lesser<>(m, n) << endl;
    24. 	// 把x和y强制转换为int类型，再使用模板函数2
    25. 	cout << lesser<int>(x, y) << endl;
    26. 	return 0;
    27. }

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  • 原文地址：https://blog.csdn.net/Mako5455/article/details/134418073