《C++PrimePlus》第8章函数探幽

8.1 内联函数

使用内联函数

#include <iostream>
using namespace std;inline double square(double x) { return x * x; }int main(){double a;a = square(5.0);cout << "a = " << a << endl;return 0;
}

8.2 引用变量

将引用用作函数参数（使用const）

#include <iostream>
using namespace std;
double cube(const double &ra);int main(){double x = 3.0;cout << cube(x) << " = cube of " << x << endl;cout << cube(5) << " = cube of " << "5" << endl;cout << cube(x+5) << " = cube of " << x+5 << endl;return 0;
}double cube(const double &ra) {return ra*ra*ra;
}

将引用用于结构

#include <iostream>
#include <string>
using namespace std;struct free_throws {string name;int made;int attempts;float percent;
};void set_pc(free_throws &ft);
void display(const free_throws &ft);
free_throws &accumulate(free_throws &target, const free_throws &source);int main(){free_throws one = { "Rick", 13, 14 }; // 最后一个值没赋值，为空free_throws two = { "Jack", 10, 16 };free_throws team = { "All", 0, 0 };set_pc(one); // 赋值display(one); // 展示display(accumulate(team, one)); // 汇总return 0;
}void set_pc(free_throws &ft) { // 要修改原始数据，不加constif (ft.attempts != 0)ft.percent = 100.0 * float(ft.made) / float(ft.attempts);elseft.attempts = 0;
}void display(const free_throws &ft) {cout << "Name: " << ft.name << endl;cout << "Made: " << ft.made << '\t';cout << "Attempts: " << ft.attempts << '\t';cout << "Percent: " << ft.percent << endl;
}
// 把函数的返回值定义为结构体引用
free_throws &accumulate(free_throws &target, const free_throws &source) {target.attempts += source.attempts;target.made += source.made;set_pc(target);return target;
}

将引用用于类的对象

#include <iostream>
#include <string>
using namespace std;
string version1(const string &s1, const string &s2);
const string &version2(string &s1, const string &s2);
// 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 << "You string: " << input << endl;result = version1(input, "***"); // 在字符串前后都加上***cout << "Your string enhanced: " << result << endl;cout << "Your input: " << input << endl;cout << "-------------------------------------" << endl;result = version2(input, "###");cout << "Your string enhanced: " << result << endl;cout << "Your input: " << input << endl;//cout << "-------------------------------------" << endl;//input = copy;//result = version3(input, "@@@");//cout << "Your string enhanced: " << result << endl;//cout << "Your input: " << input << endl;return 0;
}
// const string &s2 对应的是 "***"
// 当使用const限定符时，会产生临时变量并进行类型转换
string version1(const string &s1, const string &s2) {string temp;temp = s2 + s1 + s2;return temp;
}
// 返回一个string类的对象的引用
const string &version2(string &s1, const string &s2) {s1 = s2 + s1 + s2;return s1;
}/*错误的使用方法：返回临时变量的引用
const string &version3(string &s1, const string &s2) {string temp;temp = s2 + s1 + s2;return temp;
}
*/

对象、继承和引用

#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;
const int LIMIT = 5;
void file_it(ostream &os, double fo, const double fe[], int n);int main(){fstream fout;// 先在路径中里新建这个txt文件const char *fn = "ep-data.txt";fout.open(fn);if (!fout.is_open()) {cout << "Can't open " << fn << "." << endl;exit(EXIT_FAILURE);}double objective; // 物镜的焦距cout << "Enter the focal length of telescope objective in mm: ";cin >> objective;double eps[LIMIT]; // 目镜的焦距for (int i = 0; i < LIMIT; i++) {cout << "Eyepieces #" << i + 1 << ": ";cin >> eps[i];}file_it(cout, objective, eps, LIMIT); // 在终端上显示file_it(fout, objective, eps, LIMIT); // 在文件中显示cout << "Done." << endl;return 0;
}
// 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
void file_it(ostream &os, double fo, const double fe[], int n) {os << "Focal length of objective: " << fo << endl;os << "f.l. eyepieces" << " magnification" << endl;for (int i = 0; i < n; i++) {os << "    " << fe[i] << "    " << int(fo / fe[i] + 0.5) << endl;}
}

8.3 默认参数

默认参数的用法（取出字符串的前n个值）

#include <iostream>
using namespace std;
const int ArSize = 80;
char *left(const char *str, int n = 1); // 默认参数n=1int main(){char sample[ArSize];cout << "Enter a string: " << endl;cin.get(sample, ArSize);char *ps = left(sample, 4);cout << ps << endl;delete[] ps; // 注意new和delete成对出现ps = left(sample); // 使用默认参数cout << ps << endl;delete[] ps;return 0;
}char *left(const char *str, int n) {int m = 0;while (m < n && str[m] != '\0') m++; // 确定字符串长度char *p = new char[m + 1];int i;for (i = 0; i < m; i++) {p[i] = str[i];}p[i] = '\0'; // 最后要补上一个空字符return p;
}

8.4 函数重载

函数重载示例（取出字符串/数字的前n个值）

#include <iostream>
using namespace std;
const int ArSize = 80;
char *left(const char *str, int n = 1);
unsigned long left(unsigned long num, unsigned int ct);int main(){const char *trip = "Hawaii";unsigned long n = 12345678;int i;char *temp;for (i = 0; i < 10; i++) {cout << left(n, i) << endl;temp = left(trip, i);cout << temp << endl;delete[] temp;}return 0;
}char *left(const char *str, int n) {int m = 0;while (m < n && str[m] != '\0') m++; // 确定字符串长度char *p = new char[m + 1];int i;for (i = 0; i < m; i++) {p[i] = str[i];}p[i] = '\0'; // 最后要补上一个空字符return p;
}unsigned long left(unsigned long num, unsigned int ct) {unsigned long n = num;unsigned int digits = 1;if (num == 0 || ct == 0) return 0; // 特殊情况while (n /= 10) digits++; // 判断数字有几位if (digits > ct) {ct = digits - ct; // 要除几次10while (ct--) num /= 10;return num;}elsereturn num;
}

8.5 函数模板

函数模板示例（交换两个数的值）

#include <iostream>
using namespace std;
template <typename T>
void Swap(T &a, T &b);int main(){int i = 10;int j = 20;cout << "i, j = " << i << ", " << j << "." << endl;Swap(i, j);cout << "Afer swap, i, j = " << i << ", " << j << "." << endl;double x = 24.5;double y = 81.7;cout << "x, y = " << x << ", " << y << "." << endl;Swap(x, y);cout << "Afer swap, x, y = " << x << ", " << y << "." << endl;return 0;
}template <typename T>
void Swap(T &a, T &b) {T temp;temp = a;a = b;b = temp;
}

重载的模板示例（交换两个数或两个数组）

#include <iostream>
using namespace std;
template <typename T>
void Swap(T &a, T &b);
template <typename T>
void Swap(T a[], T b[], int n);
const int LIMIT = 8;
void show(int arr[], int n);int main(){int i = 10;int j = 20;cout << "i, j = " << i << ", " << j << "." << endl;Swap(i, j);cout << "Afer swap, i, j = " << i << ", " << j << "." << endl;int d1[LIMIT] = { 0,7,0,4,1,7,7,6 };int d2[LIMIT] = { 0,7,2,0,1,9,6,9 };cout << "Original arrays: " << endl;show(d1, LIMIT);show(d2, LIMIT);Swap(d1, d2, LIMIT);cout << "After swap: " << endl;show(d1, LIMIT);show(d2, LIMIT);return 0;
}template <typename T>
void Swap(T &a, T &b) {T temp;temp = a;a = b;b = temp;
}template <typename T>
void Swap(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;}
}void show(int arr[], int n) {for (int i = 0; i < n; i++) {cout << arr[i] << " ";}cout << endl;
}

调用函数时的最佳匹配（打印数组内容）

#include <iostream>
using namespace std;
template <typename T>
void ShowArray(T arr[], int n);
template <typename T>
void ShowArray(T *arr[], int n);struct debts {char name[50]; // 名字double amount; // 数量
};int main(){int things[6] = { 13,31,103,301,310,130 };struct debts mr_E[3] = {{"Rick", 2400.00},{"Jack", 1300.0},{"Rose", 1800.0}};double *pd[3]; // 3个元素的数组，每个元素都是指针for (int i = 0; i < 3; i++) {pd[i] = &mr_E[i].amount;}ShowArray(things, 6);// 更匹配 void ShowArray(T *arr[], int n)// 会打印出来指针指向的数值ShowArray(pd, 3); return 0;
}template <typename T>
void ShowArray(T arr[], int n) {cout << "template A:" << endl;for (int i = 0; i < n; i++)cout << arr[i] << " ";cout << endl;
}template <typename T>
void ShowArray(T *arr[], int n) {cout << "template B:" << endl;for (int i = 0; i < n; i++) cout << *arr[i] << " ";cout << endl;
}

引导编译器使用指定函数（打印较小的值）

#include <iostream>
using namespace std;
template <class T>T lesser(T a, T b) { // 函数1 返回较小值return a < b ? a : b;
}int lesser(int a, int b) { // 函数2 返回绝对值的较小值a = a < 0 ? -a : a;b = b < 0 ? -b : b;return a < b ? a : b;
}int main(){int m = 20, n = -30;double x = 15.5, y = -25.9;// 非模板函数优先，调用的是函数2cout << lesser(m, n) << endl;// 非模板函数不是最优（要进行类型转换），调用的是函数1cout << lesser(x, y) << endl;// 尖括号<>告诉编译器使用模板函数，调用函数1cout << lesser<>(m, n) << endl;// 把x和y强制转换为int类型，再使用模板函数2cout << lesser<int>(x, y) << endl;return 0;
}